2004 THX Subwoofer install

I just got my 2004 LS v8 with the THX headunit. I love everything about this car but I feel the need for more bass than the factory subwoofers can give me. I'm wanting to add an amp and subwoofer to the car while maintaining the factory subwoofers. Does anyone know how I should attempt this?

What comes to mind is splicing into the factory amp's input signal. Is this the best way, or a horrible idea?

Thanks in advance!

I just got my 2004 LS v8 with the THX headunit. I love everything about this car but I feel the need for more bass than the factory subwoofers can give me. I'm wanting to add an amp and subwoofer to the car while maintaining the factory subwoofers. Does anyone know how I should attempt this?

What comes to mind is splicing into the factory amp's input signal. Is this the best way, or a horrible idea?

Thanks in advance!
I haven't messed with this radio yet but it should be very doable. All you'll need to do is add a high level to RCA converter which can be had at any stereo shop.

Ok, I've got my aftermarket subwoofers about 99% done and it sounds great. The only question I have at this point is, does anyone know where I can find an easily accessible wire to tie my amplifier's remote wire to? The only open spots I can find in the fuse box are on all the time, not just when the car is on. Thanks in advance!