Great speaker sounds terrible in my room?

I don’t think the basic problem of having poor balance due to insufficient bass will be resolved by changing components. What would really help is to spend about $100 on a measurement mic. and download a free program called REW. This is the only way to remove the guesswork. There is a forum that has a dedicated thread on using REW. It is a very useful tool and can also help you find the best position and set up for the sub you have.

I’m wondering if a single better sub (Rel S510) would be a game changer or make matters worse.

More than 1 sub results in a better distribution of modes. If you take your new found measurement skills 😎 and look at a plot of your speakers in your room you will notice huge peaks and valleys (nulls) instead of a nice ideal flat response. The peaks obscure detail and the nulls rob you of information, this is info containing important parts of the structure of music. It is essentially lost and unfortunately EQ can not bring it back!

Adding more subs will result in many more peaks and partial nulls that serves to smooth the response. The peaks will be lower and the nulls fuller. The more subs the smoother. So try for 3 or 4 and as I explained they do not have to be huge. The REL will reach down low and the additional smoothing-subs will even out the room which you can watch happen on your computer. Using smaller subs will smooth out the low frequencies as well as big expensive subs, they just won’t reach down as low but you will now hear stuff you didn’t know was there. The whole sound is improved and by that I mean the mids and tops with a w-i-d-e sweet spot.

If you have some mates with subs see if you can borrow 2 to just test the waters. If you set them up properly I can assure you that you will soon find places to put them, like behind your sofa. There is much to be found on this on the net. Doing this will be the game changer.

No amplifier, cables or any tweaks will provide you with what I’ve described.

The lack of bass is from lack of boundary reinforcement. Do what's needed to keep these speakers because to replace them with something that equals let alone surpasses them is going to cost lots. They present an easy load and are fairly sensitive. I liked what I heard.

Something to try is to place them directly on the floor in the same position you have them now. This will allow you to hear what some mistakenly call room gain. Sure, the imaging etc. will suffer but not that much. Perhaps there is a height where the balance of sound is closer to your target.

You have bass traps which is good. Do you also have some broad-band absorbers at the first reflection points? You need to consider that all 3 axes should be treated. Absorption on either the back or front wall needed or both. Same for side walls. For the floor to ceiling axis an overhead absorber is better than any carpet but you may reject the idea so won't waste our time explaining what and how if you're not interested.

There is more to consider. If you find the bass increase to your liking when placing speakers on the floor then perhaps it's possible to mount the speakers in the kitchen area and then position your primary listening area about 38% from the back wall.  At 50% into the room you are in a partial null unless you have extensive bass traps and employ a distributed bass array.

Finally, look into a multi-sub approach. Somewhere in these threads I mentioned how this can be inconspicuously  incorporated into the decor. Huge subs are not needed. You can consider 6.5" drivers in a sealed box with an 8" square footprint which is not difficult to hide. Perhaps use one to support a pot plant, hide one behind the sofa. 2 such subs plus your REL will do more, much more , than just bass.  The entire spectrum is enhanced. Check up on the theory and application of a DBA. Also try plugging the ports for a sealed box response which makes it easier to integrate with subs.