What does a subwoofer solve?

OP,

I think that it is great that get to audition a sub or two in your listening room. I am anxious to find what you think.

According to manufacturer specs M900u balanced input impedance 34 kΩ and BHK 300 mono blocks balanced input impedance 200 kΩ. If the preamp has high output impedance that may explains the Luxman doesn't has enough bass.

While @imhififan brings up a very probable cause, I can assure you that the BHK300’s don’t lack treble, or some other frequency anomaly.

I’m using them with a very revealing speaker, the TAD CR1.  I’ve never heard the speakers sound better, even against TAD’s own electronics (though that was from models ago)

Just curious why you got rid of the BHK’s?  Especially with some tube rolling, they are an amp I don’t believe I’ll ever get rid of, and know of one person who thought they sounded better than his Constellation amps, which were also designed in part by Bascom King, but are fully solid state.  Perhaps someday an amazing GAN amp with tube input stage may better them.  
 

Brings more problems than benefits. Get a good full range and use the sub for HT only

a properly matched system will not make you aware of "missing bass" 90% of the time, but i have at least 3 pipe organ recordings that are fairly palpably loud below 20 cycles per second [locks the air in the room with a vigorous pulsation of air pressure] that NO standalone stereo speaker system i’ve heard can touch, not even Snell type As or KEF 105.2s with monster amplification. for those 3, a great sub is MANDATORY. i know of a retired doctor whose large house contains a massive digital pipe organ - to reproduce those 32’ contra bombarde pipes he uses 8] twin-12" acoustic suspension subs in massive cabinets. that amount of speaker enabled the visceral pulsating room-shaking sensation of real 32’ organ pipes to fully manifest.