Originally Posted by z3roz
I've read reviews, looked at graphs, read comments, and still have no idea.
Your old sub was a vented 15" 750 watt servo sub.
Let's pretend that Rhythmik's claim that it gets more SPL from its execution of servo than Velodyne means that your old sub was approximately equivalent to the $1399 Rhythmik FV15HP 15" vented servo sub with 600 watts.
Adding a second sub gives you about +6db than using a single sub.
So, using Rhythmiks own SPL levels at 20hz, two $599 300 watt LV12Fs will yield a "loss" of just 2.5db max spl at 20 hz.
If you didn't run your old sub at its absolute maximum you should be just fine with two LV12Fs.
Your old sub was (H/W/D): 21 x 18.375 x 20.75 inches (with grille), the LV12F is 22" (H) x 16 "(W) x (D) 20-1/2" D with grille and amplifier knobs)
So you may, indeed, be forced by aesthetics to get a pair of RSL Speedwoofers or PB1000s.
Good news is BOTH those companies offer free returns.
Start with one of each and decide which you prefer and return the loser.
If you feel you need +6db more output with the winner, simply add a second.
Sure the RSL doesn't dig below 20hz like the 12" ported subs from HSU/Rhythmik/Monoprice THX but it might give you everything you need.
The PB1000 does dig deeper.
350watts =15w16h17d= 40 lbs 10" [30hz] $399 RSL SPEEDWOOFER 10S, (extension from Sound & Vision review)
300watts =15w19h20d= 46 lbs 10" [18hz] $499 SVS PB1000, (extension from Sound & Vision Review)
500watts =15w19h20d= 72 lbs 10" [20hz] $500 MONOPRICE MONOLITH THX 10"
300watts =16w22h21d= 69 lbs* 12" [19hz] $569 RHYTHMIK LV12F
350watts =15w21h22d= 62 lbs 12" [18hz] $607 HSU VTF2 MK5, (extension from Audioholics review)
First chart is the PB1000 as indicated and the second chart is from Sound & Vision for the RSL.