replace Mcintosh with Luxman

Hello,
I would call both manufacturers and ask them if the other brand works well with their product. Also, talk to a dealer who is a dealer for both of those products. They might say it works great or give you a better option. Make sure you let all the people know as much information as possible. Type of music you like. Size of room
Do you have subwoofers and how are they connected. 
Treated or untreated
The cables that will be connected 
All of this information makes a big difference 
I hope this helps. 

I question how much of a difference you'll get with replacing a nice pair of mono blocks with another pair of nice mono blocks?  Maybe the preamp would be worth a second look?

I think it would absolutely be the right way to go for me. :)

However, if you are happy with the Mc sound especially in the high frequencies you may find the Luxman to be warm by comparison.

Luxman's are neutral, extended, and have a magical midrange.

As vinyl_rules implied, when a stereo amp is operated in BTL (Bridged) mode, its ability to drive low impedences is halved, b/c each channel's xsistors is "seeing" only half the load.  While the M900U is indisputably a magnificent stereo amp, I'd throw up a caution flag on intentionally buying a pair for monaural use.  Look for mono amps that are purpose-built as such.

I use a mc255 mac 5 channel amp for 3 channels.  Seems very nice but no one seems very happy about mac gear. Seems overated.