Several things going on. Below 20Hz (below port resonance), you can expect weird speaker noises and complaints from the driver, as the motion will be fairly uncontrolled. A sealed system might be more to your liking if real subsonic action is what you're after vs impact... and as you noted, you may be able to place it (or them) more easily/effectively.
You can (should?) peak some EQ at 25Hz, but then roll it off below (peaking HP filter). The specs have that in there (4645C_20Spec_20Sheet.pdf).
That will also make your 200w run out steam much sooner (800w would not be excessive) in the 20-ish Hz range...(a 6dB boost with the prescribed filter).
Having the sub closer to your seating location might help (can turn it down some), but could be sub (haha) optimal for other reasons.
Are you letting the 2470s roll off naturally, or crossing them over with a complementary crossover to the 4645?
Hard to guess what's going on in your room, and I'm not an AV consultant
The avnirvana folks might have some ideas for you (no association) re placement, measuring, optimization, etc... or someone here might