Dear @lldd : " BHK 300 mono blocks. The bass they provided for my Monitor Audio PL 300ii speakers was exceptional. ..................... Luxman M900u. Very happy with the new amp, except for disappointing bass performance. "
Well, both amplifiers are way different designs Where the BHK is an hybrid design with MOSFET output devices the Luxman is all SS design with bipolar output devices.
I think that you are comparing that " bass..exceptional " that in reaslity is more a colored/distorted sound coming from the BHK against the accurated and less " colored " Luxman design that between other characteristics play in pure class A for its first 12 watts that due to the Monitor Audio specs permits that you have in reality better sound with than with the BHK. Yes, I know that you like more that frequency range in the BHK but this fact is other kind of matters.
In the other side you are talking of " bass " in your speakers when the crossover in the MA comes at 500 hz and the speaker goes down around 32hz. It’s really dificult to to pin-point true bass range when the woofers/ports handle that really wide frequency range. Btw, you can play/test too with ports open/closed and the like and due that both amplifiers are so different you need to play too with little changes in the speaker position in your room: backward/foreward and its angles at your seat position you are accustom to. Even you can play too with changes in your seat position.
My take is that that Luxman is a superior amp but you are the best judge about because you know what you like.
Looking that wide woofer frequency range ( 32hz-500hz handled at the same time and those " long " excursion of the woofers say at 35hz affects directly to the remaining frequencies handled by the woofers: increment its distortion levels. ) that 500hz at the speakers crossover and other than the true bass frequency range adding a pair of powered subwoofers crossing at around 80hz using lo and high pass filters will improves the overall quality listening room/system against what you have to day and against with what you had with the BHK due that when you liberates the speakers woofers of bass frequencies below 80hz suddenly your main speakers will goes way lower in its today high IMD levels. This is the main positive contribution/improvement of subwoofers with the speakers you own and additional to that now you will have the true bass range handled by woofers and amps with dedicated units that were designed in specific to fulfill the true bass frequency range reproduction needs. Your main speakers will performs and shines better than ever, its clarity/transparency/dynamic/headroom and lower colorations will be amazing to listen it from top to bottom.Obviously you need a good integration of those subs where what you listen be a unity.
You do not mentioned if your system has a room treatment that almost always is mandatory for better quality performance levels.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.