Which component to upgrade to improve bass control?

Good footers for your speakers will decouple the speakers from the room. You will get crisp bass and way smoother mids and highs.

This comment stunned me. Maybe it's something huge that I overlooked but I thought that the bass waves being produced by the woofers and being amplified by the room were not something that could be neutralized by decoupling. If I'm wrong about this, I'd like to know, but it seems like something to do only *after* major problems involving the room had been addressed.

The proposal by hshifi — a better preamp, DAC, plus power conditioner, power cords, and a ground master — sound like throwing quite a lot of money at a much more fundamental problem. I'd bet an amount equal to their full cost that they would not help the issues you describe.

Your options are limited by the intrinsic acoustics of your room.  Regardless of the kind of room you have (dedicated or not) there will be peaks and valleys in the bass response.  Before making any purchases you might want to try a free/easy setup tweak before evaluating the more expensive option that may/may not work in your situation.  Using an RTA or SPL (can purchase apps for iPhone as well) you want to focus on the region 25-250Hz to find a position(s) where you get the most even bass response. That is where you want your listening position to be.  It is NOT about where you get the most bass.  Check out the link below:

https://www.psaudio.com/copper/article/location-location-location/

Good luck!
Gorm

Bass at the listening place is controlled through dsp. Look at Dirac as mentioned or a simple Paradigm (Anthem) ARC processor (0,5K) between your transport and DAC. When you've become more knowledgable you can move up the dsp cchain. Opens up to the best sound.

I agree with @gormdane.  I was also faced with bass issues, even after room treatments (that I needed anyway).  For background, I have a 2 channel setup with a sub and a 7.1 home theater setup in the same room so my listening spot for 2 ch. is relatively fixed (distance to TV).
After measuring the acoustics using RWE and a calibrated mic, i used Roon's DSP to tweak the bass frequencies.  (Taming a bump at 43hz had the best result.  That bump was caused by room dimensions...something I confirmed by using an online calculator.)  That improved the sound for digital however still let me with the same issues for my analog (turntable) listening.
My dealer suggested JL Audio's CR-1, an active x-over.  Before doing anything else, I verified that the bump at 43hz was present with and without the subwoofer active.  That allowed me to eliminate the subwoofer as a contributor to the bump.  The room calculations helped me confirm that it wasn't my speakers, etc.

Long story short, I was able to borrow a sample from my dealer and test it at home.  Using RWE I was able to adjust the cross-over settings and slope to tame the bass at 43 hz...still a slight bump, however much lower than w/out the CR-1.  The bass was tigher and the overall sound was much better.
I now listen to both digital (with no DSP) and analog with the CR-1 installed in my system. 

If your integrated allows you to send a signal out to a pre-process and has a power amp in, you might want to try the CR-1 if you can get a loaner.

Beautiful Speakers, I can see why you want to keep them,

IMO, it ain’t the equipment, it’s the speakers. I’m no fan of ports, but you have already stuffed them with insufficient improvement. And you certainly have tried them far enough away from walls to know the problem is from the speaker, not the room reflections.

I suspect it’s the magnet/surround/cone that is not working together harmoniously.

How old are they?

Did they sound better when new, for a few years?

Did you perhaps buy them used? If so, any info regarding age from the seller? Serial # clue?

IOW, perhaps the surrounds are not as much in control as they should/could be? Or the cones themselves have lost stiffness?

Have you asked the maker for advice?

Or, perhaps something in the crossover needs correcting/improving, however once the crossover has divided frequencies, it’s up to the drivers to sound good themselves. I’m no crossover expert, just thinking what part of the speaker is not working properly.

from a review:

" The crossover sports just 4 components, with 1st order high-pass (tweeter) and 2nd order low-pass (woofer) filters. Parts quality - Solen metallized polypropylene capacitors, a generic-looking wirewound resistor and an air-core inductor - is solid but hardly fancy, a deliberate decision by Alan who isn’t much of a believer in expensive boutique parts. Based on the results he’s achieved here, it’s hard to argue."

https://taww.co/post/183853254737/silverline-sr17-supreme-loudspeaker-review

" It has sufficient body and power down to 60Hz or so to give music real foundation, with meaningful output down to 40Hz. I think Silverline’s quoted 32Hz bottom limit is a bit optimistic (or perhaps you just need the right room), and I preferred the speaker with my REL T-9 subwoofer providing a little extra oomph. But for a great many listeners in moderately-sized spaces, this will be all the speaker you ever need. Listening to “The Elephant” from Saint-Saëns Carnival of the Animals [Tidal, Qobuz, Spotify], a track I’ve heard on some very full-range speakers (e.g. Focal Grande Utopia EM Evo), the double bass is big and present, lacking a bit of rumble that was easily provided by flipping on the REL sub. Piano left hand similarly has nice weight, never sounding diminished in scale as typically happens on small monitors. Debussy’s Ariettes oubliées song cycle from the album Paysages by soprano Susanna Phillips and pianist Myra Huang [Tidal, Spotify] is a lovely test of colors, with ethereal vocals floating above dark undertones from the piano’s lower register. The Silverline possesses suficient extension and body to bring out these contrasts with depth and balance."