I have an HD650. Not overly difficult to drive as the 800, but every bit helps.
If your dealer has/is a good tech with excellent soldering skills, then the Bryston Factory isn't req'd for this.
Oh, I really think online comments with respect to HD800 being difficult to drive are way exaggerated. It is no more difficult to drive than the HD650. AKG K701 is far more demanding in reality. Unlike the Sennheiser which require a bit of voltage, AKG K701 requires current. It's a theory of mine that AKG is sort of like ATC speakers. Very flat and responsive but rather demanding of current because of flat wire voice-coils which AKG also employs in the K701.
I think what they mean by "difficult to drive" is that it is difficult to find an amplifier that matches well with it's "sound signature". Still, I don't really agree with that either. HD800 is very revealing of up-stream components so it will show any flaw. To find components that are similarly revealing is a challenge but ultimately worth the time if you value an honest approach to listening to music without "impressionism".
This modification is way too complicated for my distributor. I have had some problems with them before. If I'll be going ahead with this, it'll be done by Bryston themselves or not at all.