can subwoofers make things worse?

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.

I recently added two Rel T9x, Klipschorns, 13'W x30'Lx9'h room. didn't really add much bass. but expansion in sound staging far surpassed my expectations. I've spent much more on electronics in the past and never experienced this level of expansion, should have done this years ago!

T series Rel's don't reach lowest freq in bass, but I've never experienced this kind  of coherence, integration with other subs, and those with dsp. With other subs constantly fine tuning sub settings with different recordings and volume levels such that it drove me nuts, gave up on subs for years.

I suspect two smaller T series would be seamless integration with your setup, as others have mentioned a single sub more difficult.  Smaller T series won't overload your room with deep bass and should provide very nice expansion of sound stage.

Doubt a sub or two will make things worse, but you have to take the time to find out where they sound best.  I currently am only using one and it’s six feet behind the listening position and only does sounds below 45hz.  You will have to figure where it rolls in and how fast the slope is. Mine is set at 12dbs, I believe and how much volume you want from it etc.

All the best.