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.