There are high and low-pass filters on the sub, so you can limit the strain on your mains. You can dial in by ear or use a meter.
http://www.dynaudio.de/eng/systems/lines/subwoofer/sub250.php?sid=430f2b96abd98#
http://www.dynaudio.de/eng/systems/lines/subwoofer/sub250.php?sid=430f2b96abd98#