Well I recently got my Firstwatt F1 clone in, and I've been burning it in steadily for the last few days.
My listening hasn't been quite as "reference" as I would prefer, as the amp has a mild grounding issue whereby I can pick up noticeable 60hz hum. It's annoying between tracks, but it's low enough that I can't detect it during music. Being pure class A and 10 watts per channel, the F1 also gets HOT. Class A dissipates 90% of the power it takes in as heat to run as linearly as possible, but that means that it's dissipating 180 watts all the time. The longer you leave it on, the hotter it gets. Its run for three straight days, and while it is essentially a small sun by this point everything inside has coped just fine. It's well-vented and well sinked, so I'm not worried.
Right off the bat, the thing that grabs me is BASS. Phenomenal tight, deep bass. It sounds like a kick drum, fo realz. A small kick drum, far away maybe, but that same visceral impact an actual drum has, just scaled down. I wasn't expecting that, but it's a nice bonus. Apparently this comes from the very high output impedance -- 80 ohms. Lower output impedance damps drivers better, but single driver loudspeakers (and earspeakers) are already very well damped. High output impedance avoids overdamping, which allows a little longer decay down low. Not enough to be smeared or muddy, mind, but enough to really sound full. I like.
What I bought the amp for was not bass, however, but transparency, and it has it in spades. It's "tell the difference between resistors" kind of transparency. I've become something of a transparency junkie as of late, so this was good news. I went through all the cliches -- "hearing things you've never heard before", "listening to music early into the morning", etc. etc. Suffice it to say, this amp is not audibly limiting the resolution of the signal in any way. It is clear as a bell, and once I sort out the grounding issue, it's supposed to be the quietest amp ever made.
Out of my Peachtree Nova's integrated amp section, the K1000s tended to have a very prominent upper midrange. It seemed overly forward and overly airy in comparison to the rest of the spectrum. That made every female vocal a little unnatural, and it's why I didn't care for the combo. The F1 has none of that imbalance, and combined with the prominent bass it has a kind of "downward tilting" character. That combined with the detailed treble really makes for an enjoyable presentation.
I still intend to pursue some SE tube amps for comparison, but in my experience the F1 is the top of the heap for SS.
FWIW, my pair is a low serial numbered set in the black box, and it's been recabled by Enigma Audio. I used the Peachtree Nova as both DAC and Preamp for my impressions, and it is phenomenal. I'm more impressed with it all the time. All components were wired up with Nordost interconnects and signalcable magicpower or digital reference power cords through a PS audio P300 power regenerator. Source was USB/optical from my iMac, or Coax from my NAD cd player.