Believe me, you are playing your system considerably louder than 55 dB. 55 dB is about as loud as if someone were talking very softly. It's like the background noise on a quiet day in a suburban neighborhood. 65 to 70 dB would be relatively quiet as well, that would be like soft background music at the doctor's office waiting room.
As for the Hsu and Rythmik subs, at 40 Hz and above, they have about the same capability. Since the Rythmik is tuned a bit deeper, it will have some more deep bass headroom, not a huge amount, but a significant amount, and it depends on what tuning mode you run any of these subs in. I would not run the FV15HP with only 1 port open, it loses too much headroom to make that worth it. If you want to compare them, here are some measurements scaled to be comparable:
Hsu VTF-3 mk5 and VTF15h mk2 and Rythmk
1 port and
2 port mode.