Hi Robert,
But a what, 1/2 inch long pre-in pre-out shunt? wow, they must have some really special ears....
So here I come with 
Nap.
Thanks Nap
You made me think of the most obvious culprit, which I overlooked.
This B60 was acquired from a studio and was in pretty bad shape cosmetically, probably sat through hours if not years of smoke, beer fumes etc. Both the volume and input selectors are noisy and should be cleaned or replaced so I can imagine those shunts, which I completely forgot to clean.
Oxidization is one of the most detrimental contributors to bad sound and if not air-tight all connections should be cleaned once in a while, this could also apply to the modular boards in Bryston amps.
I’m almost convinced now that this could have contributed to the irritability of the upper frequencies, which are the first victims of dirty connections. I’ll give the B60 another whirl, expecting the best.

Robert