It has a stepped attenuator and $405USD worth of high end V-Caps. They are not on the Decware website but can be ordered. To my ear, UFO or not, the V-Caps make a stunning sounding Zen. Going back to 2009 I've been ordering V-Caps in the all Zens I've brought into my place.
IMO, they are key parts... If you can get Steve to use Clarity TC caps in the last stage of the power supply that makes an amazing difference vs electrolytics.
I think the power issue is a personal decision, big and high quality SET amps are usually 3-stage and cost a lot more vs lower power 2-stage amps if the quality of parts are the same. I'd have built one by now if price wasn't a HUGE issue. I mean, I can't have an increase in power and a decrease in sound quality, so I was looking at many thousands of $ in parts, and some might say an intrinsic decrease in sq due to the 3rd stage, but with good parts this is minimized.
What you gain with more power is more SPL before clipping, as the Omega speakers can handle more than a few watts. How many depends on the music, an RS5 can actually handle a lot of watts, like 50+ if it doesn't have to handle anything under 400 Hz. With bass heavy music run full-range maybe as little as 5-10 watts. The larger drivers can handle more power, so especially with the 1.5 way speakers I can definitely see wanting a larger amp. It just depends if you want the SPLs. If you frequently run your small amp near it's limits then I'd definitely consider a more powerful amp.
Another option to achieve higher SPL vs a larger amp is to limit low-end frequency response of the speaker, there's a few ways to do that... Even a very low xo around 60 Hz makes a big difference and might result in clearer sounding speakers, I say might because depending on how it's done it's likely to result in detrimental side effects. With most tube pres and SET amps you can usually change a coupling/output cap value and get a 1st order roll-off, speaker level cap, DSP, etc...
