LED dimmable lights in audio room?

The room had its own dedicated power subpanel, and the lighting was supposedly on a completely isolated line.

The operative word being, "supposedly"!   

For the "I've lost count of how many times" time, there is no such thing as a completely isolated line. The closest you can get is batteries. Even then the batteries must be disconnected from the line. Even then the isolation is only partial, because RFI is everywhere. You want a completely isolated line, build your house inside a Faraday cage, with its own nuclear power plant. With just the stereo. No lights. No nothing. And with the power plant somehow isolated......    

Short of that: there is no such thing as completely isolated!   

Good stuff, guys....thanks!

Every dimmer switch and every led bulb is different. If you find the quietest set you can help them by adding ferrite chokes to the wiring at the lights.
I have them in my basement. No noticable degradation to the surround sound system there. If you can’t find a quiet dimmer just replace with a SPST.
A Trifield EM100 will help finding the noise.

The best lighting is of course the lava lamp.

I use Philips Hue lights through Wifi in my room and haven't had any problems---they were super easy to install and give off very cool effects.