Tube amp Low end Bass question

I have tube amps and dual subs.  I use the subs speaker level inputs.  The signal is sourced directly from the main speakers terminal via an additional length of the same speaker cable used for the main speakers.

Is anyone doing this with great effect?

Yes.  I am using two relatively REL T5i subs with single driver speakers from Coherent or Omega in my SET amp rig.  Works great IF you:
1. carefully match your gear
2. spend a little time to properly position and adjust the sub response.

Do you absolutely need two subs?  No.  But a pair will provide a more even and balanced bass response and make it easier to tune for "full range" listening where you can hear the lower notes but not the subwoofers themselves, as opposed to a bass effects-heavy presentation like you often hear in bad Home Theater setups.

+1 on duals subs!

Those Omen's will sound amazing with a tube amp.  You need 15w to 18w to drive them well with plenty of headroom to hit 105dB for music.  

The MC275 is a lovely amp but you will find a SET will sound heavenly in comparison and your speakers are more than efficient enough to be driven by a SET in any normal size room.  

Two subs are not essential but it will improve balance.  Key is getting the crossover point and placements just right.  I have a SET amp and two 10" subs paired with standmount speakers.

I've been using 2 RELs for years with a few different main amps (current amp is a Dennis Had single ended little gem) and they're fabulous. Bought both RELs "used but mint condition" for relatively little money and can't imagine not having 'em. Klipsch Heresy IIIs with a REL Q150e (10") and Q108MKII (8"). 

I really like to crank it sometimes, I am a little hesitant that 15-20 tube watts may not be enough with the ZU's. My current SS amp is 80 watts and it does not peter out and feels there's plenty left. That is why I was thinking the 75 watt 275 may be the trick. 

Is the Quicksilver 60 Watt Monos SET?

QS are push pull!

You could consider trying a SET from one of the retailers who offer it with a generous return policy. If it is inadequate, return it. Usually you are risking $75 in shipping charges.

I doubt a 300B SET will deliver enough power but an 845, 211, or maybe an EL34 will give you enough power.  

As a fall back, he MC275 is a nice amp. 5w less in power will have no impact on your headroom with those speakers. The Zu Omens are really easy to drive. The issue is, if you don’t like it you are usually risking a 20% restocking fee. At least that is what the retailers I have asked will charge me.

Any SETs you suggest trying out? Isn't there such thing as a large watt set? I know Decware has a 40 watt stereo amp.

To some extent, it depends on how you employ the sub.  If you are using something like the REL subs, which runs the main speakers full range and only adds low frequency support, then you will still be taxing the SET amp with having to deliver a lot of its power to play bass.  But, if the subwoofer employs an active crossover so that the SET amp and the main speakers are not being used to provide deep bass, less power is needed for the SET amp.  If you really want to play your system at very high volume levels, look for something that cuts out the main speakers from playing deep bass.

I don't know if SET is really the way to go with any headbanger kind of system.  There are some SETs that employ tubes like the 845 or 211s, but, these amps are pretty expensive, if built correctly, and very scary to own and operate, if built cheaply.  Of course there are varieties of pushpull tube amps that deliver more power.  I personally don't like most of the very high powered pushpull amps, but there are decent sounding mid-powered stuff.  To find something that will work, you MUST actually audition the amp with your particular system.

There are also output transformerless amps (OTL) that sound quite good.  Typically, these deliver very dynamic and in-you-face sound, so they can perk up a lifeless sounding system, but, having heard Zu speakers, that kind of extra liveliness is really not needed.  OTLs are still worth looking into.  

There are some relatively high powered SETs but the more power the pricier they get.  What is your budget (the MC275 is $5750)?  Do you have room above that?  Stereo or mono-blocks?  

I sell SET and Push-Pull units for Art Audio but my stuff is a bit pricier than Mc and is relatively low power.  I can help steer you toward something that is a better fit though.  What part of the country are you in?  

Also, the above comment on 845s and 211s being kinda scary in a SET stems from them running REALLY hot.  I agree.  Quality is critical.  You want separate transformers for the heaters and audio circuits and placement of internal parts has a big impact.  Designed right they only run a bit hotter than a push-pull.  Designed poorly it is like having a space heater in your room.    

With 211's and 845's, to get the higher power output, the voltages on the plates of the tube have to be pretty high--meaning properly insulated wire and good electrical construction practices, which are not always found on cheaper amps.  Also, the output transformers have to be of quite high quality to benefit from the use of these tubes and the proper transformers are quite expensive.  These tubes do run hot, but, that is not the big concern that I have.  I do agree that layout is particularly critical for the 211 and 845 amps because of the heat and high voltage issues.  While I have heard some very nice 211 and 845 SET amps (Viva and Wyetech come to mind), to be, the best sounding SETs are the lower powered ones utilizing 300b, 2a3 and 45 tubes.  

The McIntosh MC275 is a pushpull amp, not a single-ended triode (SET) amp.  The current production model is, to me, pretty lousy sounding (mushy and lifeless sounding at modest volume levels) and the construction is shockingly shoddy.  With McIntosh, it is the older stuff that one wants.   

Hmm. I'll have to do some further digging on the 275. Maybe a nice Mark V is the way to go. I've heard a Mark VI...sounded nice...however only for a few minutes and not in my own system.