Ah that's very interesting to know! Apologies, I'm really quite new to all this.
The RF-7 II goes down to 30Hz ± 3dB, and so I thought I'd need something to go much lower, like below 20Hz. In particular for movies? But do you mean smaller subs, while they go lower than the RF-7 II's they don't do it very well? Sorry, I'm not too sure what exactly the difference is there.
I'll have a look into the SVS SB2000 too, and as you say if I get that I can cut it off where the RF-7 II's take over better.