Before I had my AVN5435 navigation system, I had a stock JBL 6 disc system with aftermarket infinity two-channel amp, infinity reference speakers in front, and infinity basslink.
I hooked it up after the stock JBL amp using the speaker-line input. Surprisingly, it sounded quite good. The important thing is to ground the amps down very well. As far as sound quality, it may only be slightly lower than my current setup.
The reason why I upgraded was because I wanted the new functionality of the aftermarket setup. But for just listening to CD's and using the USA spec auxilary adapter, SQ was more than adequate with the aftermarket amp after the stock amp.