Why didn't you get a second FV15HP since if their performance about is equal, you could have saved $800 ?
I was curious. I saw the PB13U highly touted as the Daddy-O, I had the money, so I got it.
It was not because of any problem or dissatisfaction with the FV15HP. Was it worth it? To my ears, no. A second sub was worth it, but I can't say the SVS is $800 better than the Rythmik.
But, now I know. If I had 2 FV15HPs, I would still be wondering if the SVS or something else was better. It's the same thing as an external amp. Does one make a difference? Not to my ears, but it answers my curiosity.
I suggest you're in that spot now. Most likely, you won't hear a difference between your final choices. Are you the kind of guy who can make a decision and be satisfied? Or will you always wonder if "the other" is better?
If you'll wonder, get one of each. You can play with them and decide for yourself over the years if you think one is better. In the meantime, since their difference is a very small percentage of their total performance, you can mix them and be fine.