This is the answer from Florian. I’m not sure it will answer your question. I suggest you email to him directly. He is very responsive.
In fact the monaural mode also exists for M1.1, and it is different compared to bridged mode. In monaural mode, a single amplifier channel is used and it is driven by the full power supply. That's a typical mode of operation when connecting low impedance speakers. The bridged mode is more appropriate for high impedance speakers which requires high voltage excursions.
In term of noise, I do not see any penalty to use bridged mode as long as the amplifier section itself is quiet enough. it is correct on the other hand that in bridged mode, each amplifier sees 'half' of the load and needs to be able to drive low impedances. For the M1.1, it is not recommanded to connect speakers with an impedance below 2 Ohm in bridged mode, while loads below 1 Ohm are supported in monaural mode.