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

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