I looked it up in the old head-fi thread... It is a pure tube amp, class A/B push-pull. Speaker taps (which I use for, well, my speakers) have 25W per channel (bit ambiguous as iFi state that the power output "is equal to a 25W solid state Amplifier when driven into real speakers using music". Probably a bit less than 25W if you'd measure it driving sine waves into a resistor, since it uses four EL84 tubes). More than enough power for my Focal Aria 906's in my 5,5x5m living room, though. The headphone out uses the same tube amp, and is rated at 8V/7000mW into 16ohm. According to iFi it never leaves class A when driving headphones. It's quite a brilliant little amp. Has a great built in phono stage, and DAC (basically the DAC module from the iDSD micro).
That's exactly what I hear.
Thanks for your replies! Sometimes you need someone to temper the GAS... I think I am doing pretty good. With the Arya, this amp is dead quiet. Using my old Sennheiser HD700, I hear a faint noise though. (Not a problem; the Stereo 50 also has a lower-powered headphone out, rated 250mW, for IEMs and such, and no noise there).
When I further pursue a desktop setup, I might go with a Burson amp. The flexibility of having three gain stages is attractive, and I like the class A topology. I don't think I want to go into extreme clinical sounding territory...