The isolators are basically transformers, which only pass AC. Due to the size & impedance of these transformers, there's a low frequency rolloff (as well as a high frequency one, but that's generally way out of audible range).

It will affect a sub if you're feeding one from one of these signals; I currently have an isolator in my Civic because I've not got time to fix it properly... I used one pair (through the isolator) to feed front/rear speakers, and the rear empeg out with no isolator to feed the sub - as the sub has a low-pass on the amp input, you don't hear any whine from it anyway.

Hugo