First of all, The best way to know what your in room frequency response is, is to measure it properly. I too second the REW recommendation before you spend on bigger, badder subs.
Second, a bigger sub won't undo a suckout unless its coupling nature is designed to do so (IE a dipole open baffle sub EQ'd flat)
Third, back to your Songtowers. The Salks use dual 5" drivers. While the bass tuning gets impressive extension out of them, I really doubt they would be capable of much so-called "slam". I'm sure they're nice drivers but they've never really struck me as a speaker designed for output. While above ~250hz they probably have plenty of output, IMO below that range or so you normally want more raw output simply because bass tends to IMO be a bit more dynamic.
To do so you need more displacement and with passive speakers, more amplification. This is especially true in larger rooms. What might have a lot of slam inside a car would need much more displacement inside a home due to dispersion loss. that doesn't even factor in whether your bass is flat or not. In car you may be getting +8db boost at 50hz-140hz for all you know. Not particularily natural but occasionally enjoyable depending on what you're listen to. You really need to isolate what it is you're missing because right now it seems you're on a wild goose chase.
Much of what some people perceive as midbass is actually closer to being upper bass and I think that's relevant to note. All the output in the world from 45-90hz matters not if what you desire is output around 180hz for example. Have you ever tested the frequency response of songs that you feel are inadequate on your system?
