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James Tanner did clarify on this in the past. It is the way it was designed.
Don't let the BHA-1 being powered off fool you, this SS amp is on equal footing with the mighty EC Aficionado. An incredible SS amp for sure! No harshness or etch from this bad boy. I can't really find any fault with the BHA-1 as it seems to just let the music through untouched. It won't hide the poor mixing/mastering of some music or the fact that your headphone has a nasty treble spike or other deficiency. This is closer to a wire with gain than the GSX mk2 IMO.
Try it with the hd650, mate. I tried quite a few SS amps in the past. This gives out bass detail and punch on a different level. And I thought I would not never go back to SS amp after going tubes.