Thanks to everyone's contributions to this very interesting thread. I should be receiving my PSii in the next two weeks hopefully, which will be my first tube-based headphone amplifier, and I intend to use them with my HE-500s as I read that this combo has some great synergy. I'm currently driving the HE-500s off my Audiolab M-DAC but I'm looking for a bit more bass and punch (dynamics) which the PSii should be able to provide.
I am also interested in tube rolling with the PSii, but after going through this thread I must admit that I am more confused as to the myriad of tubes to try out? I have had a hybrid-tube speaker amplifier in the past which I unfortunately sold when I converted my stereo setup into a home theater. Back then it shipped with a EH 6922 tube in it which had great bass, but it sounded a bit recessed in the mids and a tad muddled in the highs. I replaced that tube with an Amperex 6DJ8 which had great clarity and extension but was a bit too mellow'ish with rock based music (guess I'm too used to that typical solid state edginess?), but I loved it with classical and jazz music. I am thinking of trying out the E88CC Brimar as recommended in another thread and then a couple of 12AT7s (Amperex, Raytheon, Sylvania and Ultron).
I was also wondering whether tube selection should be based on the headphone type it will be used with? As orthodynamic headphones typically like voltage swing as well as current output I was wondering which valve type would best meet this criteria if it in fact relevant at all? From what I've read it would seem that 12AX7 have the highest gain, but lower current output compared to 12AT7s.
Thanks in advance for your feedback!