Graeme, just looking at the picture and not having access to the manual(if you have it the connections should be described), I'd be inclined to agree with Jay that the other input labeled "XOVER INPUT" is the one intended to be used when a receiver is doing a crossover. So switch the cable to that input, which then bypasses the internal sub low-pass filter and makes that setting on the sub control irrelevant.

-----------------------------------

Enjoy the music, not the equipment.