RF Supression on Solid State Audio Amp (Kustom Amplifier)

Does anyone have advice on how to reduce RF interference in audio amplifiers (Kustom 200 Amplifiers)? Normally, I don’t find these amps to have such egregious issues rejecting RF, but they are being used in a recording studio on the 3rd floor of a building approximately 100 yards away from a Police Station, Fire Station, and various 5G cell towers. The amps do not exhibit the noise symptoms in my basement shop (but I can recreate some of that noise transmitting via my handheld ham radio near the amp). If you’re familiar with Kustom amplifiers, you’ll know the chassis are not totally enclosed. The two sides and top are 100% open and it lives inside a wood cabinet. I’ve tried retro-fitting a shield with copper tape to the chassis, which attenuated some of the noise, but not nearly enough and it is not reliable. My understanding is that the shield must have absolutely no gaps and I think the 1/4″ input and speaker jacks definitely leave room for stray signals to get in.

I’ve also verified the issue persists when completely disconnecting the pre-amp, leaving only the output section of the amplifier connected (Board PC703). This leads me to believe that adding additional filtering to the pre-amp input won’t resolve the issue.

I’ve seen in a few Mr. Carlson videos that RF can exist on the line voltage supplied to the amplifier, so I’ve ordered some ferrite beads for the AC cord and 0.01uf X1/Y2 safety capacitors to put across the line inside the amp in case the RF is actually being injected via the power supply.

What are some other options to help reject RF in such “a highly RF environment”?

As a side note, the recording engineer has been in the same studio for many years and has not had issues like this with their other gear (plenty of vintage tube amps / consoles / rack gear / etc…). Only their three Kustom amplifiers are affected.

Thank you