This is how it went.I am using the AUX input on the 990.I run analog cables from my computer to the 990.When I hit the sweep button on the computer it sends a signal sweep to the 990 AUX input and the signal is sent to the amps.My thinking is that a full range signal sweep is crossed over by the bass management section of the 990 and only signals below 100hz (my chosen xover point) are then sent to the sub output.It seems the roll off is by the driver at aboput 450hz and no lopass is processed by the 990.