I was in the same boat as you.
I wasn't happy with the volume I was getting out of the factory DSP amp.
So I followed this thread.....
http://www.xoutpost.com/bavarian-soun...dsp-audio.html
The bypass is pretty easy, but you need at least 2 amps (10 channels+1 mono channel for a sub and an electronic xover to do properly)
It's a big job to do. Costs a ton if you get top quality gear and is a pain in the ass to hide everything so it looks factory. Depending on your equipment you'll probably loose your spare too.
I ended up putting a 4 channel (Orion 8004) to power some Focal 165K2P in the front doors and I used an monoblock (Orion 2500D) to power 2 Focal 33KX (13 inch)subs in a custom fibreglass enclosure where the spare tire goes.
After everything was said and done, I had a little engine noise as well as rattling all over the back of the hatch. After getting a flat tire and waiting for a friend to pick me up to go get my spare I decided it wasnt worth it.
It was 10 times louder, it was hidden from sight but the engine noise and missing spare overshadowed any benifits.
So everything came out. Back to factory.(except I left the door speakers in)
I bought a small Audison amp and a smaller Focal 27V2 (11 inch) sub.
The new amp is mounted beside the DSP amp and the sub is in a small sealed enclosure untill I get some time to build a fibreglass one in factory location.
It doesn't shake the car at full volume anymore but it sound much more balanced now.
So what I'am getting at is.............
Upgrade your FRONT speakers first
Then add a sub and amp to the factory stuff.