There's a pretty good discussion going on about that in
this thread, although not specific to your question, it might help you decide.
Keep in mind you are comparing a $3300 sub to a $1300 sub... not hardly a fair comparison. Personally I would totally expect the Fathom to outperform the SVS Plus/2 for that price difference. I'd be one upset dude if I bought one and it didn't.
I'd keep the SVS ...OR... save a bunch of money and buy another SVS. I cannot imagine it being worth 2-3 times the cost of the SVS.