I've owned one for a while now. It replaced a Manley Jumbo Shrimp. It is an outstanding headphone amp and I doubt there is much it won't drive in that department - it certainly handles my Sennheiser 600's and Beyerdynamic T1's without any issues. I used mine as a preamp to drive solid state amps ( Bryston 7B SST2's )- I have a friend who did the same with Bryston 28B SST2's until recently and another friend who uses his with Manley Snappers. The 300B isn't the tubiest pre you've ever heard but with those Snappers in triode mode it's a pretty warm-sounding combo - much less so with the Snappers in ultralinear mode. It's quiet. As far as tube-rolling, I did a fair bit - eventually found that EML 300B's were the ones I liked best - also tried the EH 300B's it came with (ick ), Shuguang Black Treasures ( better ) and Psvane 300B-T's ( These were a little better than the Treasures ). Using NOS Mullard GZ34 rectifiers and RCA 5691 red base in lieu of the 6SL7GT's. The Chinese and Russian rectifiers and preamp tubes it comes with are junk - when one of the OE Chinese 5AR4's blew, it sent my preamp in for service. None of us ever found that rolling the 0D3's made any appreciable difference. FWIW, EML is of the opinion that the Manley pushes it's 300B's pretty hard - they describe it as "a preamp on steroids " and they suggest going to the beefed-up version of their 300B's - I haven't tried those.
Soundwise, lovely big soundstage with impressive depth and a nicely holographic presentation of vocals as one would expect from 300B's. Detail is quite good but not as good as either my Esoteric K-03 direct into the amps or my Sphinx Labs Project 8, the SS preamp which relegated my 300B to headphone-amp duty. Single-ended only which turned out to be a huge issue for me when I changed amps as the ones I ended up with couldn't be run in mono-block configuration with a single-ended pre, only in stereo. This led me to compare it to my K-03 going direct and realizing from this how much resolution I was giving up, eventually replaced the Manley with the Sphinx which is an outstanding preamp.
Hope this helps - anything else you're wondering about, ask away.
D.D.