Which component to upgrade to improve bass control?

One of the beautiful things about multiple subs, the more subs the less the room matters, and the less it matters where in the room the subs go. It really is a rather pointless time waster to measure room modes. Basically, every time you move the sub it also moves the modes. Some of them however almost always wind up being in the same places. Like, almost regardless of the room the bass will be stronger hear a wall, even stronger in a corner, and strongest where walls meet floor or ceiling.   

Basically, placement with one sub is a nightmare until you finally give up and pick your compromise. With two it gets a little easier but still a major compromise. With three, not so bad. By the time you get to four it hardly matters where you put them the bass will be so good you won't believe it. 

Springs go under the speaker. What the speaker is on hardly matters. Everyone worries about stability. Springs wobble but they really do nothing to change stability, the balance point of the speaker is the same with or without springs. 

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.