Yeah, Jules,
It's a worry, everything you do has impact, it's confusing.......
But even with the Soraya, my best amp, there is no doubt the BB sounded better. This was with a modded Squeezebox, mind, which has a highish output impedance and is known to have issues driving even the 40K Zin of a good amp.
It could be that with an unmodded SB the difference would be less marked, since there is a buffer (with about 6dB gain, like the BB) inside which would negate some of these gains.
Cheers,
Hugh