switched hu, sound cuts with volume jump

Ok I followed your advice and checked the system from the ground up (haha...) anyway, I disconnected inputs and speakers and everything was fine up to max volume. I then began adding speakers one by one and testing to high volume, still ok. Interestingly what I found doing this was very strange. The two back speakers are fine, but in the front it gets weird. The front has a tweeter/woofer presumably with a passive crossover in the door. When I hooked up the right front set of lines to the right front output on the amp I got full sound in the driver and passenger front of the car...when I hooked up the last set of cables nothing changed, except the amp went into protect! Then I tried taking the bad set off, and putting the good cables into the left front output this time...same thing, sound out of both speakers up to full volume, and would go into protect if the "bad set" were plugged into the right front output. I am even more perplexed now because the install looked professional from what I could tell and I guess I didn't have the car long enough to be able to tell that the front left and right speakers were both playing only a single side of the stereo signal. Is there a reason for this? Is this a common wiring mistake, and where would be the best place to fix it? I can't find forum history that matches this scenario. Thanks!

I should add, if the final set of speaker wire were plugged in it wouldn't go into protect right away, and it wouldn't change the audio, it would go into protect over a medium volume threshold as before.

hmm. that is pretty strange. probably what happened was a speaker wire got pinched in the past and someone was really really lazy. so instead of running a new wire they just connected the left and right side to the same wire. they probably left the other wire hanging and during the install of your new radio it somehow came in contact with ground or something else. so, i would take the opportunity to just do it right and run a fresh set of wires for your front speakers. problem should be solved

Cool. Thanks again for the help, without it I would have screwed something up! Thought you guys would find the end result pretty unusual and funny.

Yes, I agree check or replace the wires to the front speakers for the latest problem.  The orginal problem could also occur if the unused speaker connections from the new radio are shorting out when you mount the radio.  Good luck!