can subwoofers make things worse?

Yes it sure can if the sub is not time aligned/in phase. I strongly recommend highpassing the speakers (cutting the bass of the mains) through an active crossover as the timing can be corrected. 
 

those small subs don’t play loud and deep. They tend to cut the low bass when pushed hard with digital perfection circuits. Erin’s Audo corner has measurements of a bunch of them on YouTube and probably his website too. Worth watching before you buy. If you have the physical room a good 12” is the way to go was they can always be turned down. 

It's all about what you want.

I believe the REL and the SVS offer easy to return trial periods. One is a sub-bass speaker and the other is a subwoofer. Get both of them at the same time and do your own side by side comparison. It's said they both offer useful phone setup support.

With its dramatic frequency roll off the REL is easily more forgiving of placement at the cost of articulately deeper frequency heft and air. ....Set it and forget it.

If the SVS is properly positioned within your room the flexibility of the remote controlled application adjustments will provide smoother speaker integration, EQ presets and volume control all from your listening position. ....Fun.

All the best with your search.    

The SVS SB1000 Pro would be a better choice than the 3000 Micro and isn’t all that much bigger, goes lower, and is $300 cheaper.  Just another option to consider.

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!