Okay, so me and my band were playing a show last weekend. Our guitarist plugged in his amp (Marshall MA head w/ cab) and it wouldnt turn on. So I thought "It's gonna be a good idea to unplug his stuff and plug mine in to see if we have power or not" So thats what we did. My amp (Line 6 Spider 3 150w) came on, and freaked out. I feel like there was a power surge that totally screwed us.

The question is, now, The Marshall MA head will come on, but no sound will come out, so the fuses are fine. and the Line 6 comes on and all there is is just a constant hum/beep pushing out of the speakers. My fuse is also fine. I felt like this forum was the best place to ask. What could have happened, and is this fixable? Both of our amps are still under warranty but, we are kinda sketchy on whether or not Marshall covers power surges, because we've heard that they're real picky.

dude, take it to an electrician... that stuff is not userfriendly to fix, and the tubes can kill you even without being turned on - they store A LOT of energy.
Whoever is responsible for the power at the venue either pays for the damage, or marshal and line 6 will, but given both units have a multiFX unit inside, which is basically a little computer, you will probably have fried mainboards, and only the manufacturer can replace them... bad luck man, but the owner of the venue or the power supplier owe you a shitton of money. The owner is supposed to ensure your expensive gear has safe power, and the power supplier is supposed to protect from faults. I got a free tv from my power supplier when the power supply blew up (Magic smoke left the box...)