Unless I have misunderstood your post you can't just set a crossover point on your receiver and then dial in another roll-off point on one or both of your subs.
Well you can of course, but the roll-offs become much steeper and no longer represent the nominal figures you are reading on your setting dials. (in Hz or dBs/octave)
You can use both crossovers to your heart's content provided you understand that the crossover slopes are now unique to your system.
The SVS crossover roll-off is 12 dB/octave I believe. This would have to be added to the receiver's own slope. Which means the sub with the crossover point set on the plate amp probably rolls off lower than you think and with a much steeper slope.
Another other way of doing things would be to set your receiver crossover point as high you can and then just use the sub's own crossovers. That is if you want a particular crossover point. It's all swings and roundabouts in AV.
