can subwoofers make things worse?

I own the SVS Micro. That is your best bet.

Of course anything can make your sound worse if you don't know what you're doing

 But the SVS has a good Bluetooth app that lets you dial it in at your listening position

I have two REL T/5x subs I just purchased. I also have a small listening space (12x9 or something close to that). They were easy to integrate and they make everything sound so much better. Soundstage, imaging, musicality, all better, not to mention the increased bass response that, I think, is outstanding. They are fast and tight which is the sound I was looking for. Integration is easy even though there is no app to help.  They connect at speaker level with Neutrik Speakon connectors which, I think, makes integration easy and matches the input to both speaker and sub.  I would highly recommend getting two. I love the RELs but others like the SVS and the KEF. The T/5x doesn’t go as low as the other two but I get very good in room response from them. Two of the RELs are about the price of the KEF as well.

A sub shouldn’t detract if it’s setup well, and isn’t total junk. Depending on how you configurate it and the type of filters used, some subs offer a high pass filter to keep the lows out of your main speakers that theoretically could detract a bit, but it's easy enough to avoid by skipping the option of using that HP filter.

A lot of subs I’ve heard simply have the low pass crossover frequency set too high, and even more simply turn the gain up too loud relative to the mains, which can muddy upper bass/lower mids (including some vocals) and contribute to boominess. I don’t run my mains through most HP filters of a sub, and typically run them full range. Then set the sub at it’s lowest low pass crossover frequency (~40-50hz), and turn it up the gain just enough so I can barely hear it on bass heavy passages. IMO and in my setup, it’s better to use the sub to augment the bass I already have, and not feature the sub. Different playback volume levels might require some minor adjustment of the sub’s gain up or down, but it’s a matter of preference.

Room placement of the sub is also a significant factor....you’re gonna have to just experiment and see what works best in your room and to your tastes. Running two subs to help reduce room nodes is typically preferred over 1, makes placement less critical, but obviously doubles the locations you need to find.

Haven’t played with enough new subs to really recommend any of the good ones. My Dayton SUB1200 was fine for the price, but my old Definitive Technology PF1500 sounds better to me.

Good luck!

I have two Rel T/7i subs to supplement my Sf Olympica III’s and hear the same benefits as @jastralfu describes, albeit in a slightly larger room (16’w, 22’l, 10’h) -- the soundstage, imaging and presence (the apparent size of each instrument and player) was nicely enhanced and made more musical with the addition of the two Rel’s. The addition of the second Rel definately improved those sonic elements over my previous SLF850 sub (a 10") by itself and combined with the first T/7i. I set the crossovers at about 60db and the volume a little below 3 out of ten or 9 o’clock on a standard dial. This seems to work just fine for me, the subs do not call attention to themselves, the music seems to come from the Sf’s only.

It's not the quality of a sub that will really degrade the sound, it's the position, extension and whether or not you can clip excess peaks.