Joe, apologies if you have already discussed this elsewhere, but is there not an option to send the amps back to Burmester and have them converted to stereo. And ask them to set the bias as high as possible whilst they are at it
You may find it much easier to then sell the second amp, rather than a pair of monos, and keep one to run Guarneri, which will certainly be enough.
IIRC, and I often don't, you preferred your original stereo 911.
Whilst I agree with earlier comments that many smaller speakers require a good kick in the guts to come alive, and are dead sounding at lower volumes, I have not always found this relates to the sensitivity of the speaker. Seems just as much about the complexity, or not, of the cross-over to me.
Most of the better Sonus Faber small speakers have always had the attribute of sounding alive at lower volumes. And this certainly applies to any of the Guarneri.