can subwoofers make things worse?

No, a sub will not make it worse… will likely enhance a good deal, two would be best.

REL… the default choice for most. High quality and great method and controls to feather it in with your speakers so not to create a mid bass hump. 

I used a pair of REL T/Zero MKlll with Magnepan LRS+ in small room and sounded great...a poorly set up and integrated sub can certainly make things worse...

Actually, a subwoofer can make things worse if it is incorrectly positioned in the room and/or bass traps are needed and not used, the wrong crossover frequency used, or the volume not correctly set.  For most setups this means a certainly amount of experimentation is required. However, once these issues are addressed, a sub can be a big plus in a system. 

However, even when the setup is done correctly, there can be a few situations where a sub will still makes things worse. That's where the recording itself has bass or sub-bass problems. I forget the exact recording, but I can think of one I heard a few years ago where the orchestra was apparently on a poorly constructed wood stand or floor and the sub did nothing but reveal a lot of very annoying thump-thump from foot stomping. 

I tried 5 different subwoofers, 4 of them made an absolute mess, also in a small room. It's very hard to get it right, if you have limited space and options. I am still better off when I turn off my sub.