are you using the stock head unit?
an LOC (line output converter) converts a "high level" (amped) headunit signal to a lower level "source" (rca jacks) [red and white plugs like on the back on home stereo equipment] signal.
so basically, you can probably (i am not that familiar with stock bmw sound systems) take whatever input is going into your hk subs (before the amp) and hook that up to the input on your amp. maybe you would need an loc, but in theory the signal going to the hk amp is not "amplified".
ok.. time to let someone who has actually done this with a bmw hk system chime in.
oh congrats on getting good stuff, zapco and alumapro. i used to be on car audio forums way back in the day and it was, "i got a boss amp and some sony subs off ebay, how do i hook them up"