I have a 500/1 sub amp with a 300/2 stacked with their STACK-KIT-2. The STACK-KIT-2 is unfortunately discontinued but their -3 kit should easily be used with spacers in between. The BMW Business CD interfaces directly quite nicely with the JL audio amps and sounds fabulous. I'm sorry I did not cut the harness and do this many years ago. The HK amp is shameful.
2004 330Ci ZHP
BMW Performance Style 269 Wheels
BMW Performance Carbon Fiber Strut Brace
THIEL Audio PowerPlane Loudspeakers
THIEL Audio 12" Subwoofer
JL Audio Slash 300/2v2 Stereo Amplifier
JL Audio Slash 500/1v2 Sub Amplifier
USA Spec PA-12 iPod Interface
BMW Business CD stock head unit - yes it sounds great once you disconnect that garbage H/K amp!