What does a subwoofer solve?

Thanks for all of the input.  I found a local dealer who will let me borrow a pair of subs for a couple of days, so will probably take that route to start.

As to comments about doubting the Luxman doesn't have enough bass, I've seen it mentioned elsewhere, by other owners, that they too wish for a bit more in the bass.  Perhaps if I hadn't heard it right after the BHKs, I wouldn't know the difference.

But on several recordings, what was clear and dynamic in the bass on the BHKs isn't here with the Luxman.  I've had it almost a year, so completely settled by now. I made no changes to anything except swapping the amps.  The BHKs draw more power than the Luxman, so I don't think that's the issue either

One more thing: The front end is a Linn Klimax DSM Katalyst (I know I'm in the land of mostly Linn haters here, but I can live with that).  Output impedance is listed at 300 Ω.

Thanks again for all of the input.  I'll stand down on the subs for now and investigate some alternative placement of the room components, especially speakers.

One example of what I mean when I say they amp is shy on bass is Max Richter's Blue Notebooks album.  Second song (On the Nature of Daylight) just after the 1:30 mark on the song there is a sustained series of deep notes that you feel as much as hear.  With the BHKs, this was very present and audible.  On the Luxman, it's there, but barely.  This example translates to various other music.

I ditched the BHKs due to quality issues.  Three sets and they couldn't send me a pair that worked as advertised, so I gave up.