Which component to upgrade to improve bass control?

You look at it is n the wrong way. Maybe because you lack the knowledge what a miniDSP is doing for you.🙄

When saying:

very much doubt the DAC in a $250 device is going to equal my Aqua. 

If we take one of the many things miniDSP is able to do. PEQ for example, so when you have measured your listening space at your listening position with your calibrated MIC ($100).💰
And knocked down peaks and the biggest problems of your room and downloaded the generated PEQ file to the miniDSP so it flatten and solve the biggest issues in sound quality at your listening position.🎶

Then the truth is that there is NO Aqua or ANY external DSP no matter the price that is going to equal the sound quality at your listening position as the miniDSP using PEQ.🥇

You are trapped in a false belief that oh only that if I got a better this or that. As I tried to tell you can keep doing just that whole your life without getting anywhere. But that is maybe something you enjoy more than sound quality? So please go on.🔄

Another point is, if your problem is "bass blom". And you look at the basics for what type of principal your speaker is designed with. In this case a bass reflex ported construction. That is time smearing and a bloomy design in it self..🌼
So you say dear Mr bass reflex do not be what you are and you should try to be like a sealed or OB construction please.. because you are so gorgeous and I want that you please me.🧙‍♂️

You need to look at what technology you are using. And those speakers are probably not for you when they do not please you. And do not try to make them things that they are not designed for. 🤔

@invalid:

My mistake-- you are right!  

@sandthemall:

"I think it's most logical to get the room dialed in first"

I can't argue with that-- it makes perfect sense.

Implementing it, however, is another story. 

. . or can be, at least. 

It might seem logical. Then again it might be it seems that way because it is so often repeated.

As it happens the room problem is mainly due to being so much smaller than bass wavelengths. This creates room modes, which are what we hear as lumpy boomy etc bass. But the solution to room modes is multiple subs. Once you understand this and change from the old-school two locations mentality to a DBA suddenly a huge amount of "room problems" goes away.

The next biggest source of "room problems" is the room being made to vibrate by speakers physically coupled to it. This creates resonances and ringing that ruins a lot more than just the bass. When floors, walls and ceilings are all being made to vibrate by physically coupled speakers it smears and colors all up and down the audio band.

The solution to this "room problem" is to decouple the speakers by putting them on springs. This is impressively effective, as has been demonstrated many times.

If the goal is improved bass response then both of these should be done first, long before any of the other suggestions. DBA and isolation are whole orders of magnitude better than anything else you can do. The improvement you will hear from doing these goes does indeed greatly improve bass response, but also goes far beyond that one thing.