Please recommend for my next upgrade

If you haven't already, rolling to a better tube in the Maverick will get some definite improvement. I have that unit in my office system with a NOS GE 5670 ... pretty sweet.

Not sure about the Oppo SE. I auditioned that unit and IMHO it fell a little short compared to previous players - Sony XA5400ES and Tube Research Lab modified 2000ES changer. Also read a review where the basic Oppo was preferred to SE version.

You're not going to save much by getting used on A-gon. The Oppo is still new enough that not much value will be reduced from retail. Oppo Digital has an excellent return policy; so get it in for a listen in YOUR system, check it out for up to 30 days and then send it back if you don't like it. They're super fast on processing refunds too.

If you like the basic BDP-83, you could always get it modified downstream, after you save a few more dollars. ASI Tek is good ($450), and Modwright is better ($950). Good luck and happy listening!

BTW, with your system, your following step might be a power upgrade. The Volex is a very good cord, especially given the cost. Cryogenics International can cryo that cord and all your other wires for $75 a pound. Then add a Majik Buss power filter ($595) and you are much closer to where you want to be. Do some research about this little gem. It did some pretty amazing things in my system ... like make my PS Audio Noise Harvesters stop flickering and go dark!

Yes, this is a lot more than you asked for. I got a little carried away. It happens.

It seems you have a fair amount of needs/desires/places to step your rig up.

Mistakes I’ve made in the past surrounded my lack of perspective usually. Like my thinking OOOhhh, this rig is it for me. I’ll not be making significant changes soon.

But I always either did, or wanted to… well, earlier on for sure.

So instead of thinking in terms of bang for buck with this one amount of freed up pesos, perhaps thinking along the lines of where you want to wind up would be more beneficial in the long run.

I'd begin looking into power cords. Front end and back end, primarily. Source & amp. As your source could be and likely will be upgraded at some point later on, I'd focus on the amp's power cord needs now.

A pretty nice PC there will yeild sufficient enough results so that you'll feel you have bought a new amp! A friend brought over his 1080 for me to try in my system and simply by exchanging my PC for his on the rotel, he was quite impressed.

Check out a Voodoo Gold Dragon, Kimber Paladium, Audience, or Shunyata Taipan, depending upon which way you want to take the presentation.

I’ve always found once aftermarket power cords are in place the system’s voice is better revealed or allowed, if you will. So I’m big on pc’s in general as a formidable path to step up performance. They make for a great foundation or reference and will apply to other likewise devices too later on.

With those 9NTs, which I had some years ago myself, some nice speaker wires there will be a true plus. I liked Synergistic Research. I tried a few others too, Purist, Wireworld, Audience, Vampire, Monster & Canare. From where you are, some nice well heeled neutral cables there will last you thru more than a few upgrades elsewhere and allow you to hear those upgrades for what they are too!

It might not seem fun to drop $500 into speaker cables to you but it could very well contain a whole lot of wisdom.

Thereafter, looking into the cable loom including your digital link should be addressed as you can. Obtaining a good pr of main ICs should be the first move there IMHO.

Maintaining signal integrity via the wires in place is always a great idea.

RE Power strip?
It’s difficult to ignore for sure, especially if one is required, and I’d suggest you definitely look into getting something other than that Monster gizmo.

If one is needed insert a PS Audio Duet for instance, or something else around the $200 area that the Duet goes for these days.

As was said already, a better unit here will do you a better job and better here means pretty much, an investment of more than $500 if you plan on attaching all the system components. Running Springs Audio Haley is a safe bet, though they range about the $700 or more plateau..

But… even little things now can add up for you, like isolation footers for your source, pre or amp. Different spikes for them 9NTs too will help… maybe even a platform for them too!

It’d be a good idea now to enable yourself to hear the upcoming changes so speaker wires and power cords ought to gain some serious consideration.

Hope this helps.

Do have fun.

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.