I have owned plenty headphone amplifiers and still do in fact. Most of them were custom designs but to this day I have not found a more neutral amplifier than the BHA-1. I purchased it very early and so the serial number is very low as well but when I initially said it was the best match for the HD800, people laughed at me. I still maintain it is siginificantly better than Sennheiser's own amplifiers when HD800 are in question. The neutrality is inevitably the reason why it sounds good with various headphones. Bryston's philosophy was always that of neutrality in terms of response but more importantly, in terms of colour of the tone. By providing clean amplification with nothing added or taken away, it allows for other components in the system to contribute their character. Having high quality electronics does pay off here because if your source is of the same breed (not necessarily the same make make), you will hear nothing but the recording and for me, that is as good as it gets in high fidelity.
I have read somewhere on this thread that a new headphone from AKG is coming. It is something I have been waiting for years, much like a Pass labs headphone amplifier. It looks like we could see both next year! Given Pass' affection for "you-are-there" kind of an approach with slightly more presence than with Bryston amplifiers, I think it would be equally good, if not better, in some respects. But Bryston has it's own merits and so I am I am very pleased with how people have finally started to realize why it might be a highly desirable amplifier.