Guidance

There is not end to the improvements you can make. They can be in small increments or big jumps. But it isn’t as simple as plunking money down. You have to have some knowledge of where you want to go and key variables in the components and system.

I recommend reading The Complete Guide to High End Audio by Robert Harley. That will help you with overall concepts and terminology.

Then seek out acoustic music events. I spent a lot of time listening to jazz in small venues and over ten years with season tickets seventh row center. This showed me what real music sounds like. Otherwise you can go off chasing detail and bass and easily loose the magic / music from your system... like fat and salt of junk food instead of deeply fulfilling foods.

Listening to acoustic venues is required because rock and other music is created electronically, mixed electronically in unknown venues... so there is no way to know what it should sound like. When live, it is being reproduced through all sorts of electronics and speakers.... too many variables. 

@samiamsam Don't worry about too much bass. High quality subs like REL are for sound reinforcement and not just gobs of bass. Please go for a pair as it will enhance soundstaging and balance. 

 Maybe try a different speaker cable...?

You say nothing about room acoustics and/or power supply. Are you able to have a dedicated AC circuit and upgraded AC receptacle? That can improve the sound of your current system and assist in any upgrade of equipment you might make in the future.

I would try removing the Gaia feet from under the speakers and replacing them with spikes. This may couple the speakers to the floor better, which may give you a greater sense of weight. Maybe not. In my experience, how you couple to the floor makes a difference. But it is not predictable. I find that the “isolation” footers tend to clean up the sound, which is fine if you’re looking for more defined bass, but sometimes that’s not what you really want. Try it. It’s free.