The subwoofer pre out's crossover in my ES receiver is fixed at 100hz and as far as I know is not defeatable. Unfortunately, the manual doesn't provide info on the slope characterisics. To avoid stacking filters, I use the LFE input on my sub with good results in my system.
However, a workaround you may want to try is connecting your sub with speaker-level connections from your receiver, connect you main L/R speakers from the sub, and then set your mains to "large" and subwoofer to "No" in your processor to feed all LFE through your sub's x-over.
Sounds like heresy not to use the line-level connection to your sub but the speaker-level route can actually sound very, very good. It will avoid stacking filters in the signal path which can lead to all sorts of distortions.