Tks for the replies - I would guesstimate the stock wattage to the sub at around 75-100 watts. I hooked up the 12" sub in the box using the factory leads, and it was better than the stock sub, but not enough for a 1200w speaker, so I added the amp. Most if not all amps have speaker-level inputs, which is how I hooked this up. Line level signals are generally cleaner, but I have no idea how to get a line-level signal out of the B&O, abnd if there was one, it wouldn't be filtered like the stock sub lead. Running the new amp off the stock subwoofer leads makes for a very filtered frequency range. You have the crossover form the B&O system, coupled with the crossover in the amp (set to 150 hz in my case) and they multiply to give a very sharp cutoff above 160-180 hz. It's nice hearing some Chili Peppers bass riffs where Flea works his way up the neck, and the sound transitions from the sub to the mids in the doors very evenly. I like a system where 20hz, 40hz, 80hz, 160hz all play at the same SPL, so you can really hear detail in the bass guitar and drums...
Dino112 - i took the 40hz "bass boost" out completely, as it was muddying up the sound, and the gain is set to about 2/3 - full on would be too much for the rest of the system... The only thing I would do differently is spend a few more bucks on a better woofer with a bigger magnet and a rubber surround. The bass isn't quite "tight" enough for my liking, but everything could always be better, and swapping out the driver is pretty damned easy. Just like the 350 hp isn't fast enough either
Here's another thing to think about for you guys that want to replace the factory sub. Mounting it to the rear deck is a pretty bad design to begin with. You are limited to free-air subwoofers that are not going to kick the back of your seat like a sealed enclosure. Also - even the stock little sub would rattle the crap out of my doors, brake light, etc. which can be kind of annoying. Having a proper enclosure that's bolted down to the floorpan means that the vibration is sent into the chassis, not the rear walls and doors of the car. Less rattling, more bass.
One small detail - remember to renew your AAA membership - cause the way I mounted it, I can't get to the spare tire - whoops!!!