Finally got my hands on the SR60s, and ran through a huge range of stuff to at least compare the two amps, low-gain setting, bass boosts off, all source material in 320kpbs or FLAC.
While the Penguin has a slightly higher quantity of bass, I would say the C421 has it beat hands down in impact and quality/clarity. After listening to the two, I'd say the Penguin sounds more midrange-ey/honkey/congested. The C421 seems more neutral, although possibly tending towards a little bright - though that could be the not-quite-broken-in borrowed SR60s, as well - to my ear, and clarity/instrument separation is better.
Another thing I noticed, is that while the AD8397 in the Penguin supposedly has a lot higher output, the C421 is LOOOOOOOOOUD. LIKE REALLY *******ED LOUD if you want it to be. Not to say that the Penguin can't push things to unlistenable volumes, but...damn, and zero noise or hiss when the thing is cranked to maximum.
A few standouts were
Depeche Mode - Freestate (Remastered): Amazing separation, I could feel the drivers moving with the drums, and barely any clipping (considerably more on the Penguin)
Tyr - Hold the Heathen Hammer High: THUNDEROUS INTRO, I had to check to make sure the bass boost was off, and zero clipping (unlike the Penguin)
Serena Ira from the Puella Magi Madoka Magica OST: Amazing cello solo, sounds very natural and airy, much more of a "honk" on the Penguin
Joy Division - Atmosphere: This track has great...atmosphere. You can really feel the drums, rather than just hearing them, and Peter Hook's bass line stays wonderfully separated
Blind Guardian - Fly: I...wh...*drool*
The bad? Well, $170 for one, although, I'm pretty pleased at the moment. I could ask for a snazzier enclosure design, as well, but this is quite nice and small, so portability is not an issue at all, not to mention much, much sturdier than a mint tin. My one real gripe would have to be having the volume control in the middle, rather than off to one side.
While I'll be sad to see the Penguin go, since it's a cool collectable and all at this point, I can safely say that the C421 sounds fantastic, and I'll probably forget all about the Cmoy after a very short while.
Anyway, that's my halfassed "I've never actually reviewed anything before" review.
(I cannot wait to listen to this thing when I get my MS1is back from Martin.)