How to add an amp to stock dsp head unit?

[528Coug]

Had the guys at the local stereo shop add new speakers and a 12 inch
sub with a separate Alpine amp. Sounded better than the stock system.
But when I went back to add another amp for the rest of the speakers, they
couldn't make it work. It would either boost the highs and not the lows or vice
versa. So, how do you add an amp to an existing dsp head unit?
I really don't want to change out the stock head unit.
Has anyone done this? If so, how did you do it?
Your detailed description would be greatly appreciated!
Go Cougs!

[jllphan]

This place has all the stereo info you could want concerning your 5er. Scroll down to Stereo....{HERE}