What does an impedance-matching issue "sound" like?

Also, I confirmed before purchasing the second amp from Luxman that they could be run in mono configuration using the above setup so I am confident it is setup properly. 

Thanks guys this helps a bit.

I may be confusing bridging and parallel, depending on how it was described to me. 

The setup is basically diagrammed here, aside from the multiple speaker taps and the fact it is a MQ300 and not an MQ-88uSE (but they should be set up the same way)

https://onahighernote.com/blog/productfaq/connect-luxman-mq-300-mono-block/

As Al mentioned, there could be a number of factors contributing to what I'm hearing. I'll continue to run in the new amp and see how things settle down over the next couple of weeks.

Thanks guys, this is all a little confusing to me as both the speaker mfg and the dealer who sold me on running 2 Luxmans said there would be no issues. 95% of the time this sounds amazing, and I do hear the benefits of a more powerful sound and better separation between channels. But that fuzzy sound in some of the midrange on some material is what brought all this up. Hate to think that I'm stuck with these amps now if all I've done is decrease the quality of my sound. 

Also regarding tube amps and the Rivals, they are voiced with Border Patrol SET amps which are used to demo the speakers at all the shows, etc. So I don't think tubes are the issue, but it's the 3 ohms into a 6 ohm speaker part that has me concerned. 

@georgehifi this Luxman is a push pull triode amp ... and to be more accurate I don’t want to make assumptions for the manufacturer so I cannot say they are voiced with the BP amps but they are definitely known for being paired together at shows.

Al,

So if I am running the amps in this configuration to the 6 ohm speaker, you are saying based on JA's measurements and the information about the Luxman amp that it could be having a negative effect on the performance? Is it possible there is any damage being done to the amps or speakers? I was told by both the dealer that sold me the 2 Luxmans and the speaker manufacturer that this would not cause any issues with the equipment. 

Thanks!

With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

This is more than likely contributing to the issue since one amp has about 300 hours on it (and the tubes) and the other new one has about 15. 

Again I have to come back to the fact that while there is a lot of discussion about what should and shouldn't be done bridging or paralleling tube amps in general, I have specific instructions directly from Luxman on how to do this, and the Luxman distributor specifically approached me with this idea.

So while there may be many amps - even most amps - that this is not recommended for, I have specific instructions from Luxman for this amplifier, so I'm not clear how the overall configuration could be an issue. Nonetheless, this thread does bring up some interesting questions.

Thanks to everyone, and @atmasphere I am glad you chimed in (was hoping you’d be lurking here :) I am using a Backert Labs Rhumba Extreme preamp with 2 unbalanced outputs (L 1 and L2 going to the left Luxman, R1 and R2 going to the right Luxman - pictures in album linked below).

Also, all 8 KT88s have held bias steady around 485-ish since I first checked them last week.

I have had some time today to disconnect all connections, run in a single stereo configuration, then reconnect everything and give it some time to warm up, etc and make comparisons.

What I mentioned hearing before - I think it could have been my ears/imagination playing tricks on me, or simply something not properly connected or loosely connected. Everything actually sounds as it is supposed to in the parallel configuration.

What I am hearing now vs the stereo comparison is a more spacious sound field, more power in the bass in particular, and generally a stronger, beefier signal. I’m listening at about 90dB.

On the downside, it seems there is slightly more distortion in the sound running the 2 parallel amps. If you didn’t do a direct comparison it’s not likely you’d hear the difference. But there is slightly more "noise" around things like vocals, acoustic guitar plucks, etc. Again, it could be a difference in the age of the tubes, etc.

What I did was put together a small gallery of documentation for this process.
https://jsqt.smugmug.com/Luxman-MQ-88uSE-Diagrams-Measurements/n-cgz5rM/

It includes the actual block diagram of the MQ-88uSE (these amps), the specs, the instructions from Luxman on running the "C" version of this amp in parallel, as well as some additional measurements from HiFi News (UK) on this specific amp which aren’t exactly matching JA’s measurements in Stereophile.

Anyway I wanted to put all of this info together as a reference for this thread and if anyone else has ideas or questions about running 2 stereo tube amps in parallel.

Thanks again for all the info!

Thanks Al, I am curious about the vertical biamp configuration just for the sake of trying out all my options to see which sounds the best. This configuration would have benefits by running twice the output power split between the top and bottoms of the speakers?

The Volti Rivals are 3 way speakers with an external crossover (currently using jumper between the woofer and mid-horn). So it sounds like I’d use the Left channel of one amp to power the mid horn and tweeter of the left speaker and the Left channel of the 2nd amp to power the woofer of the left speaker.

The benefit here could be less distortion and more power to the individual speakers, but the trade-off between the parallel config would be re-introducing crosstalk between the 2 speakers and in theory reducing the stereo separation of the mix (vs parallel config)?

Thanks Al, so this vertical biamping actually sound preferable to running the 2 amps in parallel, if I am reading correctly.

So the connection from the preamp is the same as what I am using described in my post above (both L outputs to one amp both R outputs to the other amp) only no jumpers - I use both sets of speaker taps to go to the mid/hi inputs on one speaker and the bass input on the same speaker, then do the same on the other amp.

So this also effectively doubles the power going to each speaker - how does it affect impedance and is it safer for the amp and speaker?

@almarg - thanks, I have now experimented with the vertical bi-amping. It’s probably too early to have a firm opinion but as you noted, I noticed a little weaker bass, if not a smoother mid/hi top end. Also strangely enough it sounds like the stereo field is a little smaller than when I was mono strapping. Funny thing, the song I noticed the "breakup" from my OP was the arpeggiated rhythm guitar part in Tears for Fears’ "Woman in Chains," which now in the bi-amp configuration is NOT mostly placed in the left stereo sound stage but is now spread out across the entire sound field. Very weird as I didn’t change the output config of the preamp at all.

@ieales - the Rival has an external passive 3 way crossover. I’m sending one channel now into the mid-high input and the other channel into the bass input. I’m not sure how I would implement an active crossover into that configuration as the external crossover is built specifically for the Rival and from what I understand must be used (you cannot connect a direct signal on the speaker inputs).

Here is a photo of the bi-amp connection into the Rival passive crossover

https://jsqt.smugmug.com/Luxman-MQ-88uSE-Diagrams-Measurements/n-cgz5rM/i-dFFnJ5v/A

Based on about 5 hours of listening tonight in vertical biamp config, I’ve gone back to running the amps in parallel.

The bi-amping sounded a little smoother I think in the higher frequency. But there was an odd effect on the stereo image, seemed more like 2 separate speakers than a “disappearing” stereo field. 

At first I just couldn’t figure out what was causing the effect on the stereo image. Voices seemed to be part of the mix rather than having their own center showcase. And other elements of the sound, especially the bass, seemed more a part of the stereo field than having their own pinpoint placement in the mix.

Overall it just seemed like a “stereo” versus a staging of a live in the room placement.

i finally figured that the issue was that since the bass was compromised by this configuration, the deficiencies in bass caused the sound to be less anchored overall in the stereo field.

Anyway, that’s my best guess.

As I mentioned before, reconnecting everything seemed to correct most of the initial OP issues I had with the mono strap configuration and now that I have gone back to that config, the stereo field is back to normal. Voices are better anchored in the center of the mix. Bass is tighter and more focused. Stereo field extends beyond the speakers again, and the speakers have disappeared more for better imaging.

So it’s been quite an adventure. In many ways I am back to where I started but the Fidelity has been restored and damned if I don’t know a whole lot more about this whole process than I did in my original post.

Thanks to all who helped sort this out