I was wondering if it is possible to add a new amp and subwoofer as well as keep the original JBL 8" sub working. I would like to do this so I can just remove the sub box in case I need the space but still have the JBL 8" working?
Yup that's possible and I have that in my truck. I'm not an audio expert so I took into circuit city and paid for them to do all the wiring and install. My 8" sub still works when I pull my 2 10" subs out.
That would work, yes, however, getting an aftermarket deck, with a subwoofer output, and having them make the JBL system stay working, would be a much better idea.
Line output converters bring alot of problems, no one makes a high-quality affordable one yet.