Which component to upgrade to improve bass control?

You are missing the point completely.

A DSP will have far greater benefits than downsides. Lazy audiophiles hide behind "oh adding a component in the signal chain is devastating" OR "converting AD then back DA is devastating"

But you need a calibrated measurement microphone using REW on a computer. And there is a steep learning curve.

That is what drive 90% of audiophiles away.. it is hard.. oh i just put a spring or cement block under the component.. that is easier.. even a 10 year can do that..

Ok i have probably offended some people but in my system I could get RID of components when adding a miniDSP 2x4 HD. Removed a passive preamp, and a DAC. So win win.
And digital in and only a DA conversation..
It is only the TT that uses the analog in and reap the benefits of the DSP implement rumble filter + room correction + crossover + other benefits that is greater than lazy purist that is justify their approach that they are in the analog domain the whole signal chain.. the same people are hunting after the right component/cement block/spring. 😉🤣

You can grow with your external DSP when later on getting a subwoofer.. Get a subwoofer without DSP cheaper (it is in the wrong location in your system and is limiting what you can use it for..)

My external DSP are doing seven different sound enhancing jobs that can’t a built in DSP in a sub do..

So those seven tasks will beat any spring or cement block in sound quality improvement any day. Plus you are able to CUSTOMIZE the sound the way YOU like it. Plus you can have four different presets depending on mood, album or whatever.

You will be knowing what your system are doing in the measurements and exactly where the weaknesses there is and what to target plus fix some of your room issues. There is so much to learn and to gain but I will stop here.

I do not know what is harder.. adding vibration control and swapping in and out components (in retrospective) all your life or learn and evolve. (Just trying to be brutally honest)

Keep on listening! 😍🎶

Hello,
besides the obvious of a better preamp and DAC. You need the Puritan PSM156 power conditioner. I would upgrade the power cord to the unit. This will cost $3300. This is a no brainer. The next thing is your power cords. Make sure your amp can breath. I am going to recommend the Nordost Frey power cord. This will really help get better bass. If you can swing it go to the TYR 2. The gauge and the way it is made gives unbelievable control over the bass. If you live near the Chicagoland area this store will let you try before you buy:
https://holmaudio.com/
All of this helps the sound of the whole system. Plus, For another $275 you can add the Ground Master City to the Puritan for dead black noise floor. And it’s all plug and play. I hope this helps. 

Hello,
besides the obvious of a better preamp and DAC. You need the Puritan PSM156 power conditioner. I would upgrade the power cord to the unit. This will cost $3300 total for the PSM156 and the Puritan ultimate XX. This is a no brainer. The next thing is your power cords. Make sure your amp can breath. I am going to recommend the Nordost Frey power cord. This will really help get better bass. If you can swing it go to the TYR 2. The gauge and the way it is made gives unbelievable control over the bass. If you live near the Chicagoland area this store will let you try before you buy:
https://holmaudio.com/
All of this helps the sound of the whole system. Plus, For another $275 you can add the Ground Master City to the Puritan for dead black noise floor. And it’s all plug and play. I hope this helps. 

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