@pcc67, your concern about the lower power output of the Rythmik plate amp as compared with that of the SVS would be valid IF the sensitivity of the woofers used in both companies subs were equal. They are not. Rythmik woofer drivers are unusually sensitive/efficient, so take less power to produce the same SPL.
By the way, the F15HP has a 600 watt amp (and a 15" woofer), and costs only a little more than the F12SE. I have a pair of F15HP’s (the DIY kit version---I designed and built my own enclosures), and unhesitatingly recommend them. I and others have found them to be one of the very few subs good enough to mate with even planar loudspeakers, the most demanding mating of them all! The F12G (the G for GR Research, OB speaker designer Danny Richie’s company. He collaborated with Rythmik’s Brian Ding on the paper cone version of the F12G’s woofer, which is what is in the F12G is) has found favor with owners of Magneplanar, ESL, and other high-resolution speakers.
Why are Rythmik woofers more sensitive/efficient? Rythmik designer/owner Brian Ding explains all in the technical information section of his company’s website. One additional factor is that the Rythmik Servo-Feedback circuitry is of benefit in many ways, including in the area of power amp/woofer output maximization.