I just added a subwoofer to my Base (6-speaker) stereo. It's a 8" subwoofer in a vented box with a JL 300 watt mono amp which I tapped into the rear left and right speaker wires right behind the headunit as the source. My amp takes RCA's for the input which I used JL's recommended speaker level to RCA adapters, and I used the positive wire (green) for the cigarette lighter as the remote for the amp.
Cutoff is at 80hz and sensitivity/gain is at a little less than half. No problems there and it get's plenty loud.
However every time the bass hits at a high volume (30-40) I get a noticeable crackles from the tweeters. This never happened before and it's definitely only after I added the sub.
Furthermore, while I never had any issues whatsoever from the tweeters before, it might be noteworthy to mention that they are not the factory tweeters. They are aftermarket Tang-Band Bamboo drivers (8ohm). While I don't think this has anything to do with it, there is a slight possibility that the ohms being different could cause the crackle now that the subwoofer has been added to the system...I don't know.
Any feedback would be appreciated, I'm really not sure where to look as the install was carefully done and everything works...there is just an added crackle.