Triggering Lights off Amp Output

I was also wondering if it would be feasible to hook up some kind of lights or Led's directly to the output from an amp or the input or whatever. THe reason is because when using sound activated stuff, you have to constantly adjust the threshold whenever you change the volume. It would be nice to have lights flash at exactly the same frequency that the subwoofers are putting out. Anyone ever done it?

hmmm, i see they use transformers to isolate the amp outputs from the circuit. im thinking maybe i could use a transformer in place of the microphone in those sound activated neons you can buy at the store. that would solve the problem of noise besides the music triggering the light. but i wonder how i would go about an auto gain control, so the lights would flash at the same intensity no matter what the volume i set my head unit to, you know? otherwise if its set at low volume and i turn the volume up, the input would be saturated and the light would just stay on the whole time. at least in theory?