I have a Sony Xplode 1200W 12" subwoofer powered by a Rockford Fosgate
900W amplifier. I recently moved these components from a Mits. Eclipse
to a BMW Z4. The subwoofer is barely working in the Z4 and the
installers say it's because the power output from the factory Z4 stereo
isn't as strong as the power output from the factory Eclipse stereo.
Could this be true? I thought that's what the amp was for. The
installers have suggested I spend another $60-80 on a "line driver" to
Help this problem. Does this sound right? Thanks for any help!

Did you check your gain adjust on the Rockford ? It is possible the line levels are different between the two units,. but it shouldn't be that drastic. I assume you are rca out on each, yes ?

I am not an expert by any means, but I had a similar problem with my Memphis 12's and amp, I paid like $40-$60 (dont remember it was 4+ years ago) on a linedriver and it has helped me control the power a lot better than just the gain control...

Good Luck

Haha, you bought sony xplodes, are you retarded? You must want to waste your money...

Does the z4's factory stereo have rca outputs?