But my understanding is that there is not much of a difference between inline attenuators on the input to power amp, and reducing the gain on the power amp, but I am not sure about that.
It could help to know the BHA-1 voltage on balanced, outputs and what is acceptable input range for 2B LP but I cannot find this info anywhere
Think of an in line attenuator as a fixed single "stepped attenuator" using stepped resistors, they're highly coveted but expensive but are way more precise and less prone to failure than a variable potentiometer.
So yes it would be better IMHO to have at least one of the components fully open, in this case the 2B and use the -15 attenuator to reduce its input as I explained earlier.
Both the BHA-1 and 2B are fairly robust and will handle almost any input, if I'm not mistaken.
Input sensitivity: .775v for 60W @ 8Ω
Input impedance: 50KΩ unbalanced 10KΩ balanced
The BHA-1 schematics may help http://www.bryston.com/PDF/Schematics/BHA1_SCHEMATICS.pdf
and this one..http://www.bryston.com/PDF/Other/2BLP_GAIN.pdf
Maybe James can facilitate. : )