From what I understand, adding a second sub gives you more output and smoothes the response around the room, but doesn't really play lower. So, probably getting a plus is the way to go.
Definitely call SVS to see what they say. Usually their advice is pretty good.
One more thing, is your setup right now optimized? Do you have room EQ? Is the sub optimally placed? Maybe refining your current setup would give you the results you want for free. Often the room dictates much of the sound, so changing subs might not do that much.