I received my Darkvoice today, and though I haven't had much time with it, I'm very happy with my purchase. I'd gone to my local surplus shop (Halted Electronics) and acquired a few vintage tubes, some seemingly new, some used, so I'm currently using a smoked glass RCA 6SN7 and a Sylvania 6080WC (6AS7 equivalent). I was less than impressed with the stock tubes' quality, so I set them aside and went with my tubes.
The first thing I noticed is my low impedance headphones generate a LOT of hum. I'm not entirely sure why, when I use the higher impedance HD600, nice and quiet, but my Vmoda M100 (32Ohm) sound like there's a beehive powering them. I haven't done the Fitz mod, I probably will, but I did find that leaving the 6SN7 (and ONLY the 6SN7) in the amp and powering it on for several hours severely reduces the humming to nearly inaudible levels. I believe a thread on HeadFi stated that a 48 hour burn-in with only the 6SN7 inserted (and no input or output) was recommended to reduce the hum.
I'll need to listen to the amp for more time to get a full impression, but so far I'm very happy. I'm not sure if it's the tubes or the amp, but the bass is punchy and enjoyable. The treble isn't too bright, which I rather like since it means I can listen to it longer without getting worn out or wincing at sharp peaks. I've never had a tube amp before, but the sound is definitely warmer than a solid state. I'll need more time with the amp before I have a definite opinion on the sound, but I'm already happy, and I'm glad I got the chance to get it at a good price.
I performed the Fitz Hum Mod by installing a pair of 220uF capacitors as cathode bypass caps. I am very pleasantly surprised by the build quality of the amp. Everything is nicely heat-shrink covered, the wires are well laid out, everything is nicely tacked down so the parts don't shake apart. Very nice indeed. As for the hum mod, my M100's are dead silent, and the gain definitely increased. I was using the volume knob around 12 o'clock with the M100s before, now I'm somewhere around 9-10 o'clock. I strongly recommend this amp (with the Fitz mod) to anyone wanting to get into tube amplifiers.