replace Mcintosh with Luxman

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.

Buy American!