I went from a SVS PB12 +/2 to a single HP+ & then added a slave. I asked the same thing and several people said that the PB12 +/2 wouldn't be able to keep up and would be very difficult to play well together .... I tested for myself anyways and they were of course right just couldn't keep up - I sold it for about two-thirds the cost of a new slave .
Thanks for the info. I'm starting to think I won't bother trying. Just go with the Seaton, and when I want MOAR I'll add a slave.