Please recommend for my next upgrade

Correction from my previous post: Cryogenics International treats up to eight (8) pounds for $75.

Now back to OP request regarding "biggest bang for the buck" AND $500 budget. The question seems to be "where can I spend this amount and extract the most improvement out of my rig?" Revised recommendation: tube upgrade for DAC, Blue Jeans RCA's and coax, cryo treat ALL cables/wires, install Porter Port outlet, and add the Ground Zero speaker tweak. You should still have a little left over for some new music.

After re-reading the OP, I realized I went overboard on first response. Good luck.

If you have not already tried a few tubes in the DAC and preamp then start there. The AI will work great with a cheap pair of AMperex Bugle Boy 6DJ8 tubes (I think that is what is in there) that you can get on ebay for $20. Upgrading your source and speakers would be next. You can try to get a dealer in your area to let you try a better CDP to hear if that makes a big improvement in your system.

Thanks for all the reply. Just a missing detail, my Maverick Audio is already using GE tube. Actually, I am using a Rotel preamp now, and just bought a used AI preamp, it hasn't come yet. So, since the AI is already a tube preamp, I am not sure if I still should use the tube buffer from DAC, it seems it is a overdoing .. but I will try. And I heard good things on the Ratheon tube on Maverick. I can also try different tubes on the AI preamp.
And it seems the next thing I should do is the power cord and add a power conditioner.
I did some research on the porter port .. if change the port for the power bar, would it still be better since there is a power bar in front?
For the CDP. I did some research people recommend Rega Planet/Jupiter, or Musical Fidelity. But I also read some people commeting that with new DAC like Benchmark / Lavry which has jitter reduction, chaning the CDP may not bring any improvement. I am more leaning towards to upgrade my DAC instead of CDP. It is more versatile as I want to set up a music server, and I don't want to have 2 player, one dedicated to CD and one Blu-ray. And actually, the Maverick sounds decent, but I haven't try other more expensive DAC like the PS Audio with Cullen mod, so, I can't tell. Does any one have any experience in both and can tell if that worth the upgrade?