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

Bridged amps never sound as good as the stereo amp. 

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.

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.

Mac and B&W has always been a synergistic combo. Mac warms up the speakers which can be a bit incisive. OTOH, I think Luxman makes some of the best sounding equipment out there. Can't opine on bridging but I would be surprised if the 900U, by itself, is all you need. 

Luxman in stereo..that's my vote 

My error.

I meant to say about the Luxman:

"I would not be surprised if the 900U, by itself, is all you need."

The McIntosh amp is going to sound very nice and can sound special if paired with a high resolution speaker (such as the B&W D3). However, the McIntosh has a laid back presentation in the mids and highs. While it can sound very nice, it may not be exciting enough for your tastes.

The Luxmen, on the other hand, is a very high resolution amp but also has a very full sound in the midbass and midrange. This amp would be much more transparent and neutral when compared to the McIntosh.

Depends on what kind of sound you want.

I was using a pair of McIntosh MC2301’s/C2600 and recently switched to a pair of Luxman M900U’s/C900U. I feel I gave up just a bit of that midrange tube magic in exchange for greater detail, better front to back and left to right imaging and more authoritative bass. Midrange is still wonderful. No regrets at all about making the change. Unless you have a very large room or regularly listen at avg levels above 90db on M900 would be sufficient.

Prior to the 2301's I had a pair of 601's.  Both changes were a step forward