Which component upgrade will produce more BASS?

Speakers closer to the wall or speakers moved to an adjacent wall, and then moved around untill you find the best bass... Dance with them while they're playing bass-heavy material and set them down where you hear the most bass. I think it's a room problem... not an equipment problem. You need to optimize their placement in the room, though in some rooms it may be a futile exercise. Maybe removing some of the room treatments -- or shifting them around...?

Just guessing: if you have the speakers on the short 10-foot wall, it will result in all kinds of bass dips and peaks. And I think side-firing woofers are harder to work with than other types. If you can situate the speakers on the long wall, I'd try that. Maybe set them up for relatively nearfield listening. I think that narrow room may pose a problem (also with the 20-foot dimension being an exact multiple of the 10-foot wall). It could be that no speakers will sound right in that room.

I'd fool around with the placement as I mentioned earlier, and if you can't get a decent result, you may need to use electronic equalization to get what you're looking for.

In a room like that, it could be that the stronger the system is in the low bass and the deeper it extends, the more room problems will become apparent. So merely going to more bass-prominent speakers could hurt more than it helps.

When I looked at houses with my wife, I found that the majority of homes we saw did not have even one decent room for sound reproduction. Builders/designers don't normally consider "room acoustics," unforutnately.