You've got a pretty good sub now, what is the rush?
Why not wait a little while, save up some more money, and get the Ultra? It has an incredible reputation for doing everything well at a very good price.
The Plus might spank the seismic in HT, but it may not be "remarkably" better than your Seismic in the music category. I have the SVS SB12-plus, and I have to admit while it does well for LF extension and SPL, it does not keep up with the competition as a music sub.
My philosophy is if you are going to upgrade, don't get a marginally better sub, get a great sub!!
Just my two cents...