unique sound problem - HELP!

I have a unique problem with the sound quality in my system.

The Problem: the highs in all four of my speakers have become really hissy and ear-piercing and the vocals sound like they are coming through a tube

Possible contributing factors that may or may not help in the diagnosis: I purchased a used amp (that was of higher power and more channels than the one installed at the time) that was said may or may not work. The amp only played through 2 (the front) of the 4 channels and those channels were VERY weak. I connected the rear speakers to those channels to make sure it was for sure the amp and not my speakers. It was the amp. Needless to say, I took that amp out, but when I put my old amp back in, I had the problem described above (hissy highs and tubed vocals). Also, in my eagerness of diagnosing the problem and not disconnecting the batt, I accidentally touched the power wire to the outside of the originally-installed amp. - But the fuse didn't blow, which I thought was wierd - Is it possible that my speakers somehow got a little DC from that or something?

Attempts to diagnose:I tested a lot of things and finally replaced the amp with a new one, but I still have the problem. So, I know it's not the amp. I tried a different set of speakers that I had laying around and they did not sound as bad, but they were cheap muddy speakers in the first place and have completely different sound characteristics than the ones currently installed in my car. I have switched out the signal cables and that doesn't help. I still had 17 days of my Protection Plan left on one of the sets of speakers so I exchanged them for a new set that I will test out tomorrow. If that doesn't help, though, I'm scared that it might be something to do with my HU. Is that possible?

I thought I knew a lot about car audio, but this problem is draining my confidence!

If needed, here is what is I have currently installed in my car:
Sony CDX-CA810X (HU), pair of Polk db460 4 x 6s (front door panel) and a pair of Panasonic CJ-DA6910 6 X 9s (rear deck), a Sound Storm Labs FORCE F2100 2 ch amp, with a Panasonic and a Polk speaker wired together on each channel for 2ohms, Rockford Fosgate 12g speaker wiring and Tsunami signal cables, 8g power and ground to a 1.2 farad cap, 2g to the batt and chassis ground, and then my sub stuff that shouldn't really matter: 2 Audiobahn 12"s powered by a Profile 600w monoblock. 4g power and ground to the cap, Stinger 10g speaker wires, and Monster Cable sub/mono signal cables.

Everything was perfect and HQ until I introduced the bad amp and zapped the other one.

PLEASE HELP! Thanks!

 

Yeah, I've been meaning to test the speakers on the HU but I just haven't gotten around to it. - I disonnected the speaker leads from the back of it a while back since I was running everything through the outs and it's a butt getting my HU pulled out (97 Grand Am - gotta take off a whole section of the lower dash going from the drivers door to the HU and AC controls - on top of that, you gotta put the car in neutral or into gear to move the shifter knob far enough back to pull the whole piece off). And, I've replaced my amp, but that didn't help. Anyway, I won't have the time today to do anything that I wanted with it, but I'll get to all that tomorrow and let you know what happens. Thanks for the quick reply!