Modding/tweaking my system

You keep mentioning channel imbalance. Unless there is something wrong electrically somewhere there should be none. It is very easy however to get imbalance, all it takes is one speaker one inch closer than the other, either to you or a wall or something that might reinforce, or some furniture or something that might absorb. Or even if everything else is right but one is toed slightly more or less than the other. You get the idea. Everything has to be perfectly symmetrical down to the nth degree. Which is why I always say start with the tape measure. 

If you are gonna measure anything, measure distance. Measure angles. Those you can get a handle on. Try and measure sound, you are gonna find you got your work cut out for you just trying to figure out what it is you are really measuring. Then why. Finally how. Guarantee in the end wind up learning, nobody agrees on even the tiniest aspect of any of it. Where hopefully everyone agrees if the speakers are cockeyed so is the sound.

The trick with Nobsound, most components will only need 2 to 4 springs per footer, and some only 3 not 4. This leaves you with enough springs to do 2 components per set of 4. Springs fit perfectly into. 1/4" hole. So with a 1/4" drill bit and some wood, MDF, acrylic or whatever you can make your own.

Easily the bargain tweak of the century.

Yes it is very easy to over damp and dull the sound. All these things go a little at a time. Evaluate each step. When something sounds better, listen for what it made worse. Because usually there is a tradeoff with a lot of this stuff. Somewhere. The trick is to avoid using so much that the negatives don't start to outweigh the positives.

Nobsound springs will allow you to play with loading to tweak the top end down a bit. This will warm up the bass at the same time. Mass loading, adding mass to some of the lighter components like a DAC, is another way of improving bottom end while smoothing out the top. This is all trial and error so can take a fair bit of time and usually no one thing is a silver bullet, but it does add up and you can wind up with a lot better sound in the end.

You're on the right track with what you're doing but the things you're using are getting all the poor results I would expect. That mat material you're using is more for quieting cars than audio gear. A much better material is TA-102 fO.q tape. This damps micro vibrations revealing inner detail without losing bass or dynamics. Looks expensive but a little goes a long way.

Pull your speakers out into the room. Close to the wall muddles imaging and muddies bass. Worst is the rack by the right speaker, reflections off that are bad. Use a tape measure to align the speakers absolutely equidistant and symmetrical. Then experiment with toe. You will probably want them toed in a lot more for better imaging. But this is a tradeoff, you get better imaging and less room effects being toed in a lot, but also more direct might exacerbate your bright top end. Only you can decide the right balance. 

Get some Nobsound springs and put them under everything. These will clean up a tremendous amount of grunge and smear and get you a more natural sound. Springs are sensitive to loading. By adjusting and listening you will be able to tune your system to a lot better sound. It will take some time to figure out the best combination but it will be well worth it as you will be able to tame that top end without losing anything in the process.

Bear in mind a certain amount of top end glare will always be there simply due to what you are using. But there's an awful lot you can do to ameliorate it without trying to mask it creating problems in other areas like with the mat stuff you tried.