I think the 2 watt amp is the way to go. I like the smaller footprint, single input tube, cheap output tubes, and lower cost.

The 6 & 20 watt amps will do well. They have their own positives.

6 watt - meters, multiple output tube choices, preamp out, up to 4 inputs

20 watt - dual mono tube regulated power supply, high watts, short signal path

I agree that the 2 watt may be the best choice. Unless your speakers have a very complicated and/or power-draining crossover with the configuration and distances you are within I think you should have enough headroom. And the clarity and intimacy of the 2 watt Zen sound is a wonderful thing.

Thank you very much everyone

I ran a pair of Orca minis with a single UFO2, and felt the UFO2 was more than sufficient power-wise as well as sound quality. Depending on your room and preference, you may want to consider adding a sub-woofer. I ran the Orca's with Blumenstein Benthic sub and outboard amp.

Make sure you isolate it from the floor and it will help reduce the boom.

I'm in well damped smallish room, about 13' x 15' and am using the UFO2 and Orcas in a second system.  My other set up is solid state all the way.

The UFO2 has been up and running on and off for about 15 - 20 hours so it's still very much just breaking in.  It's extremely well put together and beautiful to look at.  My wife, who could care less about audio, actually like it a lot.  As to the sound, I would imagine anyone going in this type of direction already understands exactly what they're signing up for and doesn't expect to head bang w/a set up like this.  

The UFO2 and the Orcas are a VERY nice match.  You can hear deep into the recording, great tone and dynamics and extremely easy to listen to.  Even at this early stage I'm quite happy!  This combo will play louder than I'd want if I had friends over and we were trying to have a conversation.  More power (and probably a sub) would be recommended if you want to rock out a lot - and for that I'm not sure I'd go the Orca route even w/a sub, but I don't have any actual experience so that's a total guess.

I haven't tried the Betsy's yet.  They're supposedly about 2.5db more efficient (89 v 92.4 from what I've read), so I expect a bit more volume out of them.  Will report back on that.

I could see an extremely efficient Klipsch or Altec type set up getting plenty loud on the 2 watts the UFO puts out.  I don't have room for that.  I still would love to hear anyone who has tried a UFO with the new Omega Super 3HO Monitors or the Omega Compact Alnico Monitors.