I can't tell you much about the last 2 amps you mentioned, but I'd bet money the M2 uses the same transistors found in the F5. I'm jamming a lot more power through the same devices Nelson used and they're very reliable. You'll never have problems with that M2, and if you do, the parts are easy to get. I'd call it extremely reliable.
The F5 has a bit more provision for voicing and dialed to it's least distortion figure is cold and analytical. The M2 avoids that with the autoformer gain stage and no feedback. I'm getting a 32 wpc out of my F5 for 92dB/V Focals and it's plenty muscular. You're not going to reduce it to smoke driving it too hard, and it's very doubtful you ever would with your speakers.
The only possible down side with the M2 would be hum. Not all of Nelson's amps are dead quiet and the M2 uses the same transformer and bipolar 24V supply you find in every PP First Watt amp. Usually they're real quiet. Mine, however, has some very mild hum. I did build it myself though, and Nelson arranges his innards differently than I did.
The McIntosh will give you a slightly mellow sound, the Ampzilla is a powerful and excellent sounding amp and the designer was one of the Audio greats. The First Watt are low powered Class A amps. Never have heard the First Watt, I would go with the Ampzilla, and this is coming from a Mac guy.
Given that you have those Zu speakers, First Watt is definitely the way to go.
I recommend an F7, those 20w go a long way, and it will be more resolving/less rounded sounding than the M2 (which I have been told is more like the aleph 30 I had than the F7). I run it into DeVore speakers, not as efficient as your Zus and I never have an issue with volume in my large room.
I had an Ampzilla...... very pleased with it. I gather you sit away from the amp..Ampzilla has a cooling fan that was not intrusive to me, but it might depend where you sit in relation to it.
The M2 has been my only amp for the past 3 years, and paired with my Spendor 1/2s and AI Modulus 3B (or Quicksilver Pre), it is an astonishingly good sounding component. It does everything well, imo, and will reflect accurately what is upstream, whether it be the source or pre, etc. It creates 28W and will rock your Zus, guaranteed. The one caveat is that Nelson Pass specifies its noise w/o signal at 500uv, whereas the other First Watt amps are slightly quieter. The music mine makes, though, outweighs this minor fact, though...it is a keeper.
First watt with your speakers should pair very nicely. Class A with those speakers is the way to go.
Biggest mistake I made was selling my GAS ampazilla amp. Tons of power, quiet never came close to pushing its limits.
I think your Zu Omen Def speaker can easily go with tubes. My tube amp Cayin CS-100A KT88 has about 70watts p/c and is very dynamic and clear with a little bit warmth. With your efficient speakers you will be able to make party. It is not necessary to have 200 or more watts. The quality is more important than the power.
Any of the three would be nice. That said, the 3 times I've seen Sean demo Zu's it's always been with Pass gear. That would be a strong endorsement for the First Watt.
I'd find out if any Pass XA30.8 available around you. You'd have everything and more for these Zu...
I use a First Watt F6 with Proac 1SC. I second it for Nelson Pass amps. Class A Mosfets ,with lots of bias juice sounds incredible. IMHO.
I use a PrimaLuna Dialogue Premium Preamp with a M2 and Tekton Double Impact speakers and couldn’t be happier!
i had the m2 with double impacts..dumbest move ever to sell it..perfection... i also had a sit-2 and j2..allllll amazing..im about to re-buy a sit-2...the most perfect solid state amp ive ever heard...
@migueca At this point I would have to assume that after 5 months that you have chosen one of these amps. My Taste would lean toward the SST Son of Ampzilla 2000
Thank you all for the inputs.
I had to put this issue on standby for a while and when I got back to it, I followed a different path.
Bought a pair of monoblocks from Temple Audio and I'm very happy with the result. Everything I was looking for, I found it in these little boxes. They output 100W in class D and the sound - signal coming from the McIntosh C712 preamp and going to the Zu Omen Def - is fantastic! Clear, powerful, amazing soundstage at a very affordable price.