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I installed an amp & sub using stock head unit & stock speakers of my 2000 Honda CR-V with the PAC line output converter. My problem is: now when I raise the volume up to about 50%, the stock speakers reduce in volume about halfway and start to sound all crunchy.

I have the PAC line out spliced into the rear 2 speaker outs. The problem is definitely somewhere with these 2 speakers. I adjusted the fade to all the way in the front - didn't get the problem. Faded all the way to rear speakers - the same thing happened with the volume. Tried only rear-left and rear-right, didn't have the problem.

I soldered & heatshrink all connections - its clear heatshrink so i can tell if anything is stripped inside - and all of my connections are solid. My sub sounds great, but it also lowers a little bit when the other speakers lower unexpectedly. I had a "dirty" install first (twist and tape connections just to hook it up quick) and it worked fine at high volumes, after I went in to rework the wires this started happening. I also spliced an extension onto the rear speaker wires where the PAC connects so it has a little more slack, using "speaker wire". Maybe this is the problem?

Any ideas of what the problem might be? Anyone ever have the same issue?
I plan on getting a real HU eventually but at this point, I just need to solve the problem for my own sanity