replace Mcintosh with Luxman

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.

My apologies I am considering m900u amp and not preamp.  The plan  is for two Luxman m900u (in mono mode) for the Mcintosh.  
Thanks to all for the input it will help me decide.