According to the manual for the HK 3385:
Subwoofer Outputs: If you have a powered subwoofer, connect these jacks to the line-level inputs on the subwoofer. The same fullrange signal is output through both jacks.
So the "subwoofer" outs are simply preamp outs. Every powered sub I've seen will have an adjustable low-pass filter and some will have line level outputs that have been high passed. If you have true full range main speakers, then the low pass filter built into the sub will suffice. Otherwise, you will likely get better results using a high pass filter (whether in a sub or separately) before the main amp/speakers.
It's unfortunate that HK did not provide 2 channel bass management. Just for reference, the Outlaw stereo receiver does.