Audio Systems: 1999 Sierra wierd bass problem, sony xplod, gmc sierra

Question
I have a 1999 GMC sierra extended cab with a stock Delco CD deck. I just installed my 2 12" Sony Xplod subs, 1200w sony amp and 1.5 farad cap from my old vehicle to this one. I wanted to keep my stock delco deck so i bought an Install Edge IEC AULOC to allow me to hook up my RCA jacks and I think I have it all wired correctly because it plays fine. I can adjust the bass with the bass knob on the stock player. However, i noticed that whenever I change the radio station or put in a CD the bass jumps all the way up until i turn the bass knob, which instantly resets it to the proper level. I can't figure what could be causing this, no matter how many times i adjust my bass it always will jump up when i do the above mentioned actions. I have an aftermarket pioneer from my old truck that I can put in if its the deck but I'd like to keep it stock if reasonably possible. any ideas?  

Answer
Jared, this is very strange, indeed.

The most likely cause would be a bad preamp in the headunit.

*On a seven year old deck, I would be upgrading the headunit very soon.

I doubt the RCA convertor is doing this, but if you use the Pioneer, that is a non-issue. It should not be too hard to get an install kit and put that other headunit in.