I recently faced this same decision. I owned an SVS PB13-Ultra for the past two years and an SVS 20-39PC+ prior to that. It's not hard to be an SVS fan, and the PB13-Ultra was an amazing all around sub. The only reason I sold it was because I use my sub for 2-channel music. Music is my primary concern as I consider my setup a 2-channel system that is home theater capable. I just like it to be very capable.
As I was not happy with my PB13 for music, I had my sights set on a pair of Rythmik F15HP's. I really wanted the dual subs for hopefully better in-room frequency response, but I was very successful getting a flawlessly flat in-room response from the PB13 just using it's built-in single band PEQ. I was about to pull the trigger on the Rythmik's until a good friend told me to look into the SubMersive. So I did...
There is no doubt that choosing between dual Rythmik 15's and a SubMersive is a very difficult decision. For me, I really wanted the dual subs and the aesthetics of 2 subs on either side of my entertainment center. My wife was really looking forward to getting rid of the large PB13 and getting the smaller pair of subs. That was until I called Mark Seaton...
Mark did not sell me on the SubMersive. What he did was helped me figure out how to integrate his SubMersive with no bass management into my dual sourced setup (a Velodyne SMS-1). What did push me into the SubMersive is its dual opposed driver configuration, and the fact that he offered me a SubMersive HP. I couldn't pass up on the offer.
Back to the dual opposed drivers... One thing that I didn't like about my SVS 20-39PC+ was that the sub was far too light for the power it had. My setup is in my living room and it has a wood floor above a basement. The SVS cylinder would literally bounce and shake the floor when driven hard for HT. The down-facing driver and light weight was a problem. Fun sometimes, but I don't want to hear my room shaking vs the actual sound. When I got my PB13-Ultra, the front firing driver and heavy box was MUCH better. I could drive the sub much harder before my floor shook. But as it was a very powerful sub, the house shook anyway.
I'm very curious as to how the SubMersive HP will perform in my room when driven hard. I'm hoping to hear more sound and less room shaking; at least for anything but bass heavy movies anyway.
Mark tells me I should see my new SubMersive HP next week Wednesday. I sure hope so as I've been subless for far too long now. I'm really looking forward to getting it all setup and listening to both music and driving it hard for HT.
So while I did wrestle with the decision, I feel like I made the right decision thus far. I'm sure I would have been happy with a pair of Rythmik's, but I'm thinking the SubMersive will be "better" for me.