What does a subwoofer solve?

My experience with Luxman was the same. That was in the late 80’s. I hated every minute with the Lux. It had no bass line , just a constant thump from the bass drum.

I can’t comment on something 40 years ago, but in my recent memory, of the past 3 years or so I’ve never ever heard the Luxman’s be bass shy.  The Luxmans I know of have extension in spaces both in the treble range as well as in the bass.  I am more likely to believe the BHK lacked treble, and that Luxman is more well balanced. 😁

But seriously, if the effect the OP describes is so severe it makes him want to suddenly add a sub I’m going to question whether this is really due to an amp swap. Room acoustic issues like adding more reflective elements in the room, or taking away bass traps are the sorts of things I’d suggest first. Next, if this really really is happening, swap your cables for some Mogami 3106s. On my Luxman I had to turn the bass DOWN after I put this cable into my system.

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