can subwoofers make things worse?

Music or movies (or both)?

Big room or small room?

Neighbors?

The first two factors will depend on what you want and how competently you can work / trial your way through implementation.

The third factor, sub(s) will never make things better! 😉

In my case two Rel Carbon Specials slightly defocused midrange vocals. I tried everything - repositioning 1000X or more, and following all guidelines with respect to volume and crossover settings. Every time I thought I had nailed the setup, I realized the subs weren’t actually engaged at all! I really tried everything I was advised to try and did nothing but experiment with them every day for a month after break in. I was able to return them, luckily.

Of course, this is just my unique experience. I think most people love them, especially at this level. The deep palpable bass was excellent and well integrated. But they also didn’t produce the dynamic and energetic higher frequency bass I was seeking for my very large room with high ceilings, leaving my B&W 802’s a bit underpowered for my space. But the most annoying thing was that smearing effect on the image at center stage. Moved to Audiovector R6 Arrete’s and these gave me all I was looking for with their dual downward firing active drivers - and I have zero desire to add subs now. Bass is now deep, defined, punchy and dynamic with my Diablo 300. Really has the palpable “growl” back in bass solos that I was missing. I don’t have that same bass you “feel” with the Rel’s, but I really don’t miss it at all, and I don’t think I was every really going for that really low end anyways.  
 

Just my experience of course, which as I mentioned I think is unusual!

It’s not just subs reproducing otherwise unheard bass that exists in recordings (and all around us generally), it’s the effect of adding realism and sort of "charging" the room with a more genuine ambience. Even recordings without many really low notes like string quartets (lowest note on a cello 64.5 hz) benefit from a full range system. Unplayed piano strings resonate and give soul to an acoustic piano. Lifelike...you should want that.

Whether or not you agree with all of it, this article brings up many points for bassic consideration.

I use a REL 7i in a small space. Once properly tuned, it fits in perfectly.