Which component to upgrade to improve bass control?

A Hegel h390 has terrific bass control. Damping factor of 4000.

Damping factor, in spite of the name, has very little to do with membrane damping because there is speaker’s own coil in series for the braking current. 8 ohm speaker has about 6 ohm coil impedance at low frequencies. Adding amp’s output impedance of 0.2 ohm (DF=40) or 0.002 ohm (DF=4000) won’t make much difference (6.2ohm vs 6.002 ohm).

Get a sub or an amp with tone and/or loudness control. 

@duckworp:

1) Yes, I could. It quickly gets expensive shipping back and forth across the country, though. I went through this routine when shopping for a high-end acoustic guitar last spring and guitars weigh a lot less than a pair of speakers!  

2) I've of course heard of the DSpeaker units but have no idea how effective they might be. I could talk to Walter. 

My impression is that amps with built-in DSP are generally more of the home theater variety. I've read that built-in DSP can have unintended  detrimental "side-effects" on SQ. 

3) I had  Schiit Loki and was unable to manage the sloppy bass. I'm not confident an amp with tone controls would be any more successful. 

@kijanki:

Your comments are way over my head but thanks for contributing. 

@cakyol:

Please see #3 under response to duckworp. 

I have great bass. I don't have a dedicated room and I even got great bass from an amp notoriously known for very modest bass (Carver TFM). 

The secret is the room. And you don't really need specialized audio accoutrements.

Now it helps to have a larger...symmetrical...rectangular room with an acoustic (popcorn) ceiling. But you can get pretty good bass with less than ideal circumstances.

Remember the room telegraphs the surfaces you have in it. Lot's of pictures behind glass, tile floors, large TV screens, glass coffee table? Expect bright, smeared sonic picture. 

One of the best things you can do if you have hardwood or tile floors is to put your equipment rack and speakers on a long rug that spans the width of the entire front wall. 
A runner that goes from corner to corner against the front wall is not only amazing for bass but timbre too. The corners are important for bass. this is in addition to a large central rug.

People who say their speakers sound better at lower volumes...or that they 'lose composure' when they crank up the volume? That's mostly the room.

In a well treated room, you should be able to crank it up big time and it should sound great and not seem too loud.

You can make a small room feel bigger to a speaker by being sneaky with your decor.

My wife and I don't always see eye to eye about decor. But I'm a creative director. I can come up with some solution she will like.

Family pictures are in the dining room not the living room. Wall art is oil paintings on canvas or tapestries or wood carvings. Lots of books and LPs help. 

The TV has it's own smaller sound system and is on the side wall...it gets covered by a wool throw blanket when I do serious listening.
The aggregate of these little things make a huge difference.