can subwoofers make things worse?

I've been around subwoofers since M&K first introduced them in mid-70s. I've used them with lots of speakers, large and small including Magnepans and KEF LS-50s, both notoriously difficult to get 'right' with subs. Two major success criteria:

1) the HP and LP filter should must be used, and set as low as possible. Typically no higher than 60Hz for music, and 80 Hz for Home Theater. The HP keeps the low bass out of the mains, improving their dynamics and power handling, by reducing their low bass load. OThis means no speaker-level connection - your preamp needs a Sub Out with bass management, or the sub itself needs line level inputs and HP line level outputs back to your power amp. The latter usually limits placement options, however. 

2) Placement and level settings will take some effort. However, same wall as your main speakers, roughly 25-40% of the way across the wall is a good starting point.

Listen to both male and female vocals first to make sure the sub isn't adding mud or chestiness, then to material with true bass, (Beyond The Missouri Sky, Pat Metheny & Charlie Haden) then sub-bass (I Believe When I Fall In Love, Stevie Wonder). Most people have never heard the pitch, tone, and texture of Charlie Haden's bass done properly or the sub-bass synth line that underpins that Stevie Wonder cut. A good sub, well placed will reveal what's actually on those cuts. 

Either the T/5 (or T/7) or the Micro 3000 are excellent choices for your system. The KC62 I would recommend if you had LS-50s.

Small rooms are always the source of the a lot of issues, as the bass wavelengths are longer than the room itself.  So is also our visually driven tendency to want to put a sub in a symmetrical location- half way between our mains for example.

So subs will excite some kind of room mode no matter what.    The issue is what do we do to mitigate that.  There are many solutions from bass traps to EQ.  My experience is that using more subs turned down low is better than one big one.  For example I would rather go into a small room with 4 cheap subs placed at different distances along the four walls of the room than one sub in corner.  

I think Duke had the right idea with his swarm product.  Also bass arrays work well, in small rooms or large, Bag End did a lot of work on that.  

So we have more trouble with clients buying one sub that we do with clients buying two.  Almost no problems with people buying 3 or 4.  With ATMOS this is a big issue, as much of the basic ATMOS info says subs in front but that doesn't really work in practice. 

I can only let you know what happened with our purchase of a set of year-old Quad ESL 57s many eons ago. They were sold to us with a Dalquist EQ-1W sub-woofer with a passive crossover at about 80 Mhz. What happened was the mid-range leaked through the sub-woofer, muddying up that glorious renown mid-range of the Quads. We did not try an active crossover or bi amp the system, which would maybe have made an improvement. My suggestion: just don't.

Kef Kc62 Is good I use that little guy in my kitchen system.

Rel is not good until S models or Carbon special

SVS is for home theater

i use JL Fathom F212’s in my big system. 
look at the JL E Sub

Remember using one Sub can make it worse easier!

it’s easier to integrate a pair of subs properly and balance out the room better