Luxman M900u stereo vs Mono setup

You can ask the OP (as noted by spinaker above) of the MY LONG LIST OF AMPLIFIERS thread. Im sure he will be happy to provide you a detailed account of his Luxman m900 Monoblocks.  They are his favorite "affordable" amp to date...and that's saying something

Hello, I own a Luxman M900u as well. I would not go running off and buying another 900u. I would consider the other gear you have connected to the amp. When inserted my 990u the mid bass and bass got better. The speed and leading edges were nicer. More real. There are so many other things that affect your mid bass and bass that my have nothing to to with the amp. I feel that the source is the most important and the amp will more than likely show the good or the bad of that signal. Given a great signal, the Luxman M900u should give you what you are looking for. But for the great signal to get to the final destination, the amp... the signal goes through a lot of other gear and cables...... so..????But if you are still wanting another Luxman M900u ? I'm considering selling mine....as I have much more gear to play with....

Room treatment, especially bass traps can make compact monitors sound a lot bigger and tighter.

I'm a big Luxman fan by the way, for much of the reasons described above, but if you haven't looked into the room, or if you inadvertently moved your speakers between amps, that may be the cause.  According to the Stereophile measurements, those speakers are 4 Ohm minimum through the mid-bass, shouldn't be any problem at all.

I certainly agree that a 4 ohm load shouldn't cause a difficulty for the Luxman amp, in theory, and that is part of the puzzle.  I have not moved speakers even a mm, just subbed in the Luxman for the BHK mono amps and I hear this difference in bass.  My room is extensively treated, including bass traps.  I use high quality cabling throughout, cables that have made every other component they've been hooked up to sound excellent.  But it just might be that this needs some sorting to get the best of the Luxman. 

I will try some speaker placement work to see if there is a new "best spot" given the change.  It would be delightful if that ameliorated the issue, because the rest of the spectrum has a lot of plusses.  I sure wasn't expecting a bass quality issue....

Audiodrom gave the m900u/c900u high praise with a score of 87%.

However, they did have this critique:

Notwithstanding all that was said the Luxman combo will most likely not appeal to anyone whose music collection favours rock’n roll. The electric guitar in Sheer Heart Attack (Queen, News of The World) is precise and full through the M-900u and the C-900u, yet it lacks ultimate bite and electrifying presence I heard through the Van Medevoort, for example. I suspect it is a kind of tax for class A as the tonal purity and textural resolution brings (almost) always along the slightest touch of transient roundness. If you like your music hard and aggressive, look elsewhere.