Tube amp Low end Bass question

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?