No, the sub crossover settings are derived from the speaker x-over settings in the 990. There is no independent sub crossover setting. To direct only the surround channels to the sub, the Front speakers would all have to be set to Large.
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Current Setup:
Samsung HL61A750 LED DLP RPTV
Marantz SR5004 AVR / Outlaw 7075 Amp
L/C/R: Ascend Acoustics 340M/C/M
L&R Surrounds: Ascend Acoustics CBM 170
SVS 25-31PC
Panasonic BD55K Blu-Ray Player
Dishnetwork VIP722 HD/PVR.