Subwoofer insight.

Probably not what you want to hear but I think you should get properly sized stereo speakers for your room. Start there. It sounds like your current speakers are not up to the scale of your room.

Next, find the right location for those speaker and then treat the room. I don’t mean use crazy amounts of acoustical treatment. You can do a lot with very simple choices you make with existing furniture/art placement. One compromise my wife makes is we have no glazed picture frames in the living room. All the art on the wall is sculptural or framed oil paintings...some framed textile art. We have a 9x12 Persian rug and carefully placed furnishings.
It does not look like a listening room and isn’t perfect. But I get slamming bass and holographic soundstage in my listening position. No need for a sub.

I own two subs that are put away. Same issue: constant adjusting of the bass levels. In fact the imaging improved without them. My stereo amp doesn’t need to compete with the powered subwoofer’s transformer.
When you dial the main speakers in...then dial the subs into the mix.
I can’t imagine that a single 10-12” sub wouldn’t suffice for a room of your size. But I tend want subwoofers to just ‘fill in’ what’s missing. Some people like room shaking bass.

See if it is possible to try the subs before you purchase or it you are able to return if you chose to do so. Feeling like you're missing something is likely to be exactly what will keep you in this hobby and chasing that sound forever. I think most of us are always trying to reach another plateau which we think will be better. I think once you get to the point where you feel no further improvement is possible and you stop even exploring the possibility, you have lost what most makes this a hobby- your interest, like adding more horsepower, etc. to you sports car, in making it better. Try the subs and judge for yourself -it's what makes this whole thing fun.

For me it's the extra bass "range" that my subs provide as my main speakers have very balanced bass to their obvious limits (about 58hz)...the subs just make it all more believable. 

The SVS 16s are much better than the RELs for half the money.

rwwear,
"The SVS 16s are much better than the RELs for half the money."
There are no absolutes in Audio, only preferences. I've owned the SVS 15 and SVS 16. I currently own REL Carbon Limited subs. I would never own SVS subs for 2-channel again.