How to accurately gauge speaker sensitivity to match with tube amp?

What should I notice in those curves? Just that they don't dip down as you described -- down to... 2 ohms? For how far across the mid bass?

Volume is fine with the speakers -- they get loud without a problem. It's that control, tightness, I'm looking for.

@atmasphere Thanks for your answer. You're adding more technical information that is pertinent to my narrowing down the candidates -- the comment that "the speaker should not go much below 8 ohms especially in the bass region" is very useful, indeed (at least, to me). 

These speakers are trials. They're going back. No cost to me because I can drop them off. The question is, "what's next." Thanks for your help with that project.

@erik_squires 

 The more variation, the more the impedance curve takes command of the frequency response….

…but look how broad the under 4 Ohms is. That’s what I’d listen for. For the most neutral presentation from these speakers you are going to want a beefy solid state amp.

Great! Very helpful. FWIW, I only looked at the curve after I knew I was hearing something and wanted to understand why. I had seen such graphed curves before, but now they are connecting with my listening experience; they have meaning, now.

@miller

The only measurement you need is sensitivity greater than 90. Then from there you pay attention to listening impressions. Then when you find people describing a sound that matches what you like look for people who are hearing that sound with tube amps…..But on the other hand, maybe you don't care about such things as musical involvement.

LOL! If I didn't care about musical involvement, I'd be a fool. I hope I've not given off that impression with my question. I'm not a specs fanboy by a longshot, and your suggestion is most appreciated. People's taste (and hearing) can vary a lot, and so sometimes that's an X factor which is hard for me to glimpse in the many posts mentioning speakers in which I might be interested. Another tool (like the sensitivity number) was what I was seeking here, but I'll remain open to the possibility that you're right — that more than just sensitivity measurement is worthless and I should read up on listening impressions almost exclusively. (And, of course, all of this is not replacing listening, but getting whittled down the number of things to which I might go to the trouble of auditioning.)

@yogiboy  Thanks— good suggestion about the taps. They're currently on the 4 ohm taps, though.

@almarg That's right — I'm trying to use specs to help delimit a rather wide range of choice. The power of the amps are similar, but I know their damping factors are very different (20 vs. 100, at least). I will look into what other people's anecdotal reports say, too. I never ruled that out, but the metric of speaker sensitivity has been very helpful in narrowing down my choices and that's why I'm looking into related metrics, now.

The dynamic jumps in symphonic music (I spent a day just listening to symphonies) were not distorting with the tubes.

Thanks, @yogiboy ! There are local audition options for those.

@yogiboy 
1.Room size?
430 sq. feet. 14x26 or so; listening on the long wall, about 9 feet away.
2.Type of music you listen to?
classical, jazz, rock
3.Budget?
4-5k tops
4.Floorstanders or Stand mount?
I'm open.

Yes, Fritz's speakers sound very good. Been trying his Carbon 7s and will try Salk SS 6M's. A lot of folks were advising trying floorstanders, too so that's the phase I'm in now.

Thanks @pesky. 1 and 2 on your list sound like the key, here. “How far” the deviance is, and for how wide a range it spans, seem like the key to making a guesstimate. Erik gave some clue in his comment above. I am now searching the web for the meaning of phase angle beyond what you stated to see how that might be applied to my search. Came up with this so far:
https://www.dhtrob.com/overige/tubefriendly_lsp_en.php
I'm not sure how good the information is, but will try to cross-check.

Thanks johno...will look for that email exchange.

From the link above [ https://www.dhtrob.com/overige/tubefriendly_lsp_en.php ] this seems helpful:

"[F] the purposes of determining tube-friendliness, sensitivity is essentially meaningless. Many tube gurus insist that one must use speakers with sensitivity ratings of 90 db or higher. This is simply not true. Many speakers with high sensitivity ratings are poor choices for tube amps, just as many speakers with low sensitivity ratings often work quite well….

Tubes like a resistive load - it’s really that simple….[F]rom a tube’s point of view is as follows: tubes like consistency. They prefer a speaker that doesn’t swing from 8 ohms to 2 ohms and back again….Tubes don’t like to pump out current at the drop of a hat. They prefer a consistent and predictable load.

Naturally it is sometimes difficult for speaker manufacturers to keep all the aforementioned variables in their ideal ranges. If phase angles must be capacitive, impedance should be high. The lower the impedance, the flatter it must be, and the less capacitive the phase angles must be. All three variables (slope of impedance, value of impedance, value of phase angle) together determine how good a match a speaker will be for a tube amp.

For this reason, the argument that only speakers with an impedance of 8 ohms or higher will work on tubes is incorrect. There are many tube friendly speakers whose nominal impedance is 4 ohms. If the speaker’s impedance is relatively flat and consistently hovers around 4 ohms, and if the phase angles are only slightly capacitive, or better yet inductive, there is no reason why a 4 ohm speaker cannot perform well on a tube amplifier. Some of our favorite tube friendly speakers are 4 ohms!"

The article concludes by affirming that both metrics matter AND listening matters:

"You can easily tell if a speaker isn’t performing well on tubes by... listening to it! Certainly we encourage you to gather as much data as possible such that you don’t waste your time auditioning speakers which are clearly not appropriate choices for tubes, but when it comes down to it folks the only way to find the right speaker is to listen to it anyway. A speaker that doesn’t perform well on tubes will typically be characterized by one or more of the following sonic traits when connected to a tube amp:

• Dual-mono imaging, i.e. the speakers do not "disappear"
• Unusually poor imaging or soundstaging
• Lumpy, bloated, or "one note" bass
• Lack of control in the bass - excessive boominess or thumpiness
• A "suckout" in the midrange
• Whenever the bass kicks in, the mids go away
• Whenever the bass kicks in, the highs get harsh and strained
• When you hook the speaker up to a solid state amp, the bass gets tighter, the midrange comes back, and the highs mellow out"

The last bullet point here tells me that the Focals are a bad match BOTH for my solid state AND my tubes -- because the Focal's bass sound mediocre on my tubes, and while the solid state tightens up the bass, the mids get grainy and the highs get harsh.

@fuzztone

I'd call Mike first see what he thinks.

I wrote him. Only had a couple things for me — advises a 4 ohm tap and to get sensitive speakers.

@ieales
I figured my amps would have the power to drive towers. I have heard the ML Motion 60's are easy to drive; and other towers, too. I suppose if I cannot find a tower that has a friendly enough response, bookshelves will be fine. I do have a REL sub that's big enough to do some of what you suggest. (Crossover, gain, and phase.)

@djones @grinnell Thanks for the suggestions!

@jchiappinelli58 I had this very thought. I was conscious of making things equal but it occurred to me that I was just "eyeballing" ("earballing"?) it. An SPL meter is on my purchase list! Thank you.

Thanks J. Chip. I could hear some locally. Not the Triton 2s?

@mammothguy No bubble burst; I knew their on-paper damping factor but reports about their abilities were pretty positive. And they do a very good job, but I’m totally experimenting with speaker choices here and it cost me nothing to learn something. But I think that damping factor and these speakers provides something of a data point going forward. Appreciate your thoughts and please take care.

@almarg Thanks. I didn’t think it was all that low, in comparison to other tube amps and QS has a good reputation for driving a lot of speakers. Thanks for adding some quality information for context.

@audiokinesis I didn’t follow the technical details in your post, but I get the final drift. There is something about the combination of the QS amp and the Focal speaker that is responsible for a lack of control compared to the Adcom, and I admit after reading your post I’m less sure it’s damping factor, but now I’m also less sure what factors to be on the lookout for that create the same issues.

@fritzspeaks @audiokinesis Thanks, all. None of the issues I noticed with the Focals were present in the Carbons. These curves were experienced by my ears as very consistent across the ranges.

@ohlala Thanks for the link to VTL. As I look at it, I see that they are pointing at damping factor again. I have to confess, as a non-technical person, I have no idea what to think.

VTL: "In our experience the system begins with the loudspeaker that will work properly and sound good in the desired acoustic space. Matching the power amplifier to the loudspeaker is usually a function of driver size, efficiency and budget. Efficiency is not the only determinant – even though they may be very efficient, many speakers with large drivers need tremendous amounts of damping factor and current capability to control the driver properly.

To get everything out of the loudspeaker that it was designed to do we suggest that you use the highest power amplifier that you can."

P.S. I listened again extensively to the Focals and Carbon today. The muddiness is still there in the Focals but it’s not a lot of muddiness; just at the bottom, basically lower than the Carbons go. For them, I let my sub do the work.

Hi Audite, I’ve done some tube rolling. I have KT88s in my Monos. Not sure why a tube roll would change the fundamentals, here. 

I might have a chance to try tektons, thanks. 

I am returning the Focals and will try something else. I'm not changing amps. 

@ieales The more I think about it the more I agree. I think this would be an experiment if I cannot figure out anything else to try that is different.

@mammothguy I’m not so sure you’re not on to something, as well as the others. This was an interesting article (1967!) which helps tamp down on whatever big debates were happening about DF back then. (Are the same debates happening today?)

It seems to affirm what both "sides" here are saying, namely that while astronomically high damping factors are irrelevant, that higher damping factors can make a difference.
From the article (link at bottom):

"CONCLUSIONS
It should be obvious at this point that the quoted damping factor of an amplifier is important only if the figure lies somewhere below 20 or so. Changing the damping factor from 2 to 20 does change the performance of the loudspeaker system (for better or for worse, depending upon the speaker). But trying to prove that a damping factor of 200 or even more is somehow better than one of 20 is pretty unconvincing because the effective difference in the practical case cited is only that between 1.25 and 1.32.

But someone is bound to insist that exhaustive tests have been made with such and such amplifier and that a very high damping factor is better than one down around 10 or 15. "The bass is just a little cleaner, just a little more natural and open" is the way the argument usually runs.

In a given situation, this may very well be true. Rs is a byproduct of negative feedback. The more such feedback that is thrown into a power amplifier circuit, the lower the generator impedance and the higher the damping factor. The point is simply that if a lot of feedback has to be used to lick the distortion in a particular circuit, fine - use it. But don’t believe that the reason it sounds good because of some astronomically high damping factor."

https://butleraudio.com/damping1.php

@atmasphere re
Thanks for correcting VTL. I'm surprised they're out there with such false information. 
I'm grateful to bury the damping thing. Clearly, it refuses to die, generally.
There may be different senses of what 'Tight bass' means. One way I take it is that a speaker emulates a string bass similar to one at a good seat in a jazz club — clarity in the fretting, the pluck, and a bass-like attack and decay of the note. What I've been hearing from my speakers, at times, is a bass-colored fog. That's at least what I mean by that phrase.

Thanks. The only Audio Physic speaker near is Audio Physic Tempo, used for about $1k.

@clhs04 Thanks so much for your post. I have some lower priced Klipsch RP 600M's and they are so easy to drive. I don't think these speakers are right for my listening area — they're kicking butt in my home theater, but their mids and highs leave something to be desired (harsh to my ears).

I've returned the Focal 936s to the shop that loaned them. After two weeks of listening, their tweeter still sounded harsh and these had some miles on them. I noticed there are a lot of Focal 936s for sale used out there, lately; I don't know what's up with that; could be nothing.

I've located some really nice Legacy speakers here in town, but the dealer (who mainly does installations) won't let me take them home to try them and so I really don't see myself going with Legacy without a trial. Their AMT style tweeter and their sensitivity both seem great for my QS but…I need to try at home.

I appreciate your mention of a couple brands I've never heard of, including Tyler. Something to look around for.

@clhs04 Point taken. Another person is also pointing out how different the better Klipsch are — I recognize (thanks to you both) that I need to understand things differently. Gotcha.

It's interesting to hear that the QS 60w Monos won't be enough to drive the 94 db Legacy speakers. Or maybe you're not saying that, exactly, but rather that the amps are not sufficiently good enough quality wise? I have very good tubes in them…

@timlub I'm in Denver.

Thanks, deCooney. The capability of the mono 60s was what convinced me to get them over the mid monos (40s) and some other good tube stuff — and so it startled me to think that even by spending that much more, they would still not be capable.

I loved the part of one video where Duddleston says, "We're always having people walking in going well can your speakers be driven with a single ended amp, 16 watts or something, and so it's nice to be able to prove that, so there's a lot of flexibility."

The QS 60s weigh 30 pounds each; big honking amps. Not as big as they get, but certainly not backing me into a corner.