I just bought a Rel Q201E and it allows you to use a high level connector in addition to low-level (both active at the same time}. If I understand correctly, I can set my speakers to large and the sub still picks up any bass below the crossover point I choose on the sub's panel. I have now effectively lowered the frequency response of my inexpensive Maggie MMGs to below 20Hz and the sub integrates so well you can hardly tell when the sub takes over.

This feature along with the terrifically quick response of the Rel for music is the main reason I chose it. Unfortunately, I don't think many subs offer this feature, but I may be mistaken.