I don't know if you've ever been told this but low frequencies below 80hz are fairly non-directional. The job of the subwoofer is to deal w/the frequencies that you can feel and handoff the ones you can hear. If you have a halfway decent sub (which your SVS may be), then the wattage on that amp is either comparable to the 4800 or possibly even more!Think about it from a logistical standpoint for a moment. The sub has to play the LFE and during a busy scene, that's going to keep it pretty busy.
All AVRs will use a crossover to send redirected bass from speakers to the sub - depending on the number of channels you may have 5-13 channels sending it information.
Do you really want it handling frequencies all the way to 120hz even assuming it could do it the same way that the speaker can (which it cannot)?
Just like speakers, the less work you give it, the better it will sound. Lowering the range would reduce its workload and focus it on its primary objective which is the lowest frequencies which take too much power on the amp and stress the speakers.
For your reference, I have a DefTech Supercube 1 w/a 1500W amplifier!