Luxman M-900U in bridged mono or McIntosh MC1.25kw with B&W 800 D3

I think that the sound of the Luxman and the Mc are so different you should listen for yourself, because you are going to come out of it with a strong opinion.

Bridging an amplifier increases power into the one channel but it also increases distortion and reduces the ability to drive low impedance loads. I don't think it's ever the preferred solution.

Pretty much this. 

I agree with bridged amplifiers usually not sounding as good as stereo ones.

Unfortunately I can't audition the McIntosh MC1.25kw, that's why I'm asking for an opinion from someone who might have compared both.

Have you heard a D'Agostino?  The Luxman is awfully close to that.

I know this is not exactly what you are looking for, but I have McIntosh MC 611’s driving B&W 802D’3. One step down from you. I previously had a Classe 2300, which is a very good stereo amp. I found the McIntosh to sound slightly on the warmer side, but very dynamic, clear, and detailed, Bass and mid range are fantastic. Highs are dynamic but not bright or harsh. I know many say McIntosh is soft and romantic, but I would not describe the 611 that way. I think maybe the newer models are voiced differently from prior models.