I will be interested in the result of this.....
I did a HU & sub install on my friend's 3 series, and we spliced the low level outs into the bmw amp, and it sounds fine, but the volume is a little low (is that what you ment by cold lol)
I'm about to put the HU & sub from my old car in my e34, and will probably go low level rcas into the bmw amp, unless this turns out well
Thanks