Your post *finally* prompted me to register! I too am a long time lurker.

I recently caught the headphone bug (thanks to those SOB's at head-fi.org

) and now drive Sennheiser HD-650 through my BP-25 headphone jack.
I too must turn the volume knob way up. But IMO the Senns can be driven quite loud regardless. One thing to get used to (if you're not already) is the preamplifier's neutrality.
If you do a search, you will find a thread (
http://www.audiocircle.com/index.php?topic=28762.msg256891#msg256891) in which JT (hi JT! I love my preamp and my 4bsst!) quotes his extraordinary uber-designer-engineer thusly:
Hi James;
The headphone output is in fact the same feed as the output to the amplifier, and is nominally around 1-2V. This is to prevent ear damage from excessive loudness levels. It is designed to require the volume control to be advanced quite a bit to give loud levels. This also eliminates an amplifier stage, which reduces noise and distortion on the headphone feed.
I hope this is helpful information. Thanks for thinking of Bryston.
Chris Russell
Bryston Ltd.
Perhaps your AKG's need more oomph.