PB10 owner; worth upgrading to new PB12-NSD?

I purchased my first real subwoofer last august, an SVS PB10 and am thinking I love the bass from it so much, I may want some more. Would it be worth selling my PB10 to upgrade to a PB12-NSD, or would adding another PB10 be better, or any other sub recommendations for under $800 if I sell my PB10? Has anyone else upgraded from a PB10 to a PB12 that could comment on the difference?