Driving Lights with Audio Amplifier
Google "music activated lights" but you'll find old schematics.

If you want only the bass, you'll need to run the signal through a lowpass filter followed by a peak detector(ie, a diode). Signal can come from a microphone or the amp's output, or RCA connector...

Then you'd use the output of that to turn the LEDs on or off with a MOSFET, for example.

If the original power supply has a voltage output, you can even reuse it.

DO NOT use a MOSFET to switch the AC mains! The switch goes on the low voltage side...