Hello BigShot,
I was also looking for an answer to this very question and i have spent searching for hours without one. All i see are useless responses like "Why don't you use a Dual Voice Coil instead" or "Why only replace the sub and not the amp".
Well i had enough and went in to Bridge the Amp, i used a JBL GTO 804 (8") single voice coil sub and the result?. Sounds like Crap, very little and distorted sound. I ended up using only one "Channel" on the Amp while i install my new MTX 300 mono.
I haven't been able to find the specs on the factory amp anywhere, i'm not sure if its really a 2 channel amp or anything else.
But there you have it, bridging wont work.
best regards
Javier