When I plug my wah in reverse (guitar in the output and amp in the input) I get some feedback. I can control it with the tones knob and the wah like David Gilmour did on the song Echoes.

http://www.youtube.com/watch?v=neZRrywIoEY (this is not me, it's just to show you what I'm talking about)

So me and my friend had fun with this but I was wondering, could it actually damage the speaker? On my friend's amp he can do it only at low volume even if he cranks his amp, the volume is still low. He has a 60 watts transistor amp With my Twin Reverb it's the opposite. Evenif I keep the amp at the lowest volume, the sound is very loud and can get louder if I turn up the volume. Anybody got an idea why?