acoustic advice resonating wall panels

Maybe add another layer of sheet rock and put some fiberglass on the back of that.

Owens Corning fiberglass specs show which freqs are absorbed for each model. I think 703 or 705 probably do what you need. Cheers,
Spencer

Do the panels themselves resonate? Or are they bouncing on the walls?

The sound pressure peaks force the panels to vibrate, the very thing that the panels are intended to tame. If you take an empty box sealed on all sides except one side and place it anywhere in the room where there's a sound pressure peak or reflected acoustic wave - while playing a test tone circa 300-400 Hz - the box will vibrate strongly. That's why you need to throw everything but the kitchen sink at the sound pressure peaks. Tiny bowl resonators, diffusers, absorbers, Helmholtz resonators, whatever, since the peaks cover a wide range of frequencies. And the peaks occur in the 3D space of the room as well as room boundaries. So the problem is severe.

I placed some real mondo traps in the corners and a mini trap behind listening position near the wall and the sound got much better with improved bass and overall sounding much more lively so am happy and ain't gonna mess with success ....at least for now

That's cool. Ethan Winer knows his sh*t! Well done.
Cheers,
Spencer

Only a specific material cannot absorb the resonance. The most important thing is how you have designed this?? And it's not so easy to design an effective panel at home. Different types of cheap & very effective panels are now available in the market. Please visit http://www.acousticmanufacturer.com/Noise-Barrier-on-Temporary-Fence-for-Construction-Site.html to have an idea about different types of panel. Thanks

Take a look at these two products...

This one is the more affordable of the two

http://www.sofsurfaces.com/durasound?gclid=CMnFtLLf1NQCFZW2wAodnS0PCQ

This one you can simply glue new sheets of sheet-rock to the existing
https://www.google.ca/aclk?sa=l&ai=DChcSEwiZpvCM39TUAhUGBWkKHVtsCjQYABABGgJpcQ&sig=AOD64_2CV...=

Regards...