Just received my Darkvoice yesterday, about a month before the estimated ship date. I've been looking at buying this amp ever since I got my HD6XX since they apparently pair so well. I'm only one day into listening and I'm already impressed at the sound that comes out of this thing. Since my NOS tube order arrived the same day as my amp, I didn't even bother with the stock tubes, so I can't attest to how those work. I immediately replaced the power tube (large one) with a RCA 6AS7G and the pre-amp tube (smaller one) with a Tung-Sol 6SN7GT "mouse ears" (pictured below).
There was no perceived hum, as many have noted with the stock tubes. The unit did develop a mild burning odor as it began to warm up over the first 20 or so minutes. This was fairly faint and I could only really smell it because I was sitting very close to the amp. After that, the smell disappeared and there have been no other issues since. I have my LP-120 turntable feeding the amp, and have only tried it out using the HD6XX and the pre-amp outs to my powered bookshelf monitors. I had previously been using the Schiit Vali 2 in this setup with a NOS Amperex 6JD8 "Bugleboy," which produced really nice sound paired with the HD6XX. However, upon listening to them with the fully tube powered OTL Darkvoice, these headphones really came alive. The music sounds beautifully accurate and detailed, and I would even venture to say that it lifts some of the "veil" commonly associated with the HD6XX and widens the soundstage. I just couldn't stop listening last night and was nearly brought to tears at one point, not from a sad song, but from the depth of sonic pleasure my ears were taking in. Of course, these impressions may have more to do with the tubes I rolled, however it speaks well for the Darkvoice being a fantastic platform for rolling different tubes into. I also purchased two other NOS 6SN7 tubes, a GE 6SN7GTB and RCA 6SN7GTB which I intend to try out as well. I am really looking forward to how the sound will develop more once I put some more hours into these tubes. However, I'm already extremely pleased with my purchase and would recommend it to anyone looking to venture down the rabbit hole that is tube rolling.
(Update 12/10/18): After rolling in the two other NOS 6SN7 I had mentioned previously, the Tung-Sol mouse ears tube still remains my favorite for the types of music I enjoy, followed closely by the GE 6SN7GTB. I've since used the Darkvoice to power a set of Beyerdynamic DT-880. For me, the Darkvoice fixed everything I had initially disliked about the DT-880, which was mainly the piercing treble frequencies. The highs still sound clean and clear and the treble doesn't roll off nearly as much as the HD6XX, but the Darkvoice manages to really mellow out that brightness of the 880s. The Darkvoice also helps bring up the mids with the DT-880s, providing me with a relatively flat sounding headphone that offers more dynamic range than the HD6XX and is a good compliment to the 6XX's very mid forward sound. Since I have the 600 Ohm version of the DT-880 this is the only amp I own that can really drive them to their full potential, which is fine because they sound great on this amp. The only problem I've encountered is that with the 600 Ohm DT-880 the volume knob on the Darkvoice does very little past 11 o'clock and it can be cranked to max without having any real effect on the volume.