14 - 90 Hz (-2 dB @ 14 Hz) with LINE IN inputs is the spec i'm seeing on that force balanced Rythmik.....that looks like some deep bass.
Here's the spec i'm seeing on the monoprice....Frequency Response (‑6dB) 16 ~ 200Hz
Peak spl not so sure....but, if the monoprice will hit 110 db (you've seen that CEA 2010 chart) and the Rythmik hit a 100 db....what would i miss out on if my listening levels for music fluctuate between 85 and 95 db.
Rythmik's response window is pretty much nonsense. Any sub can maintain that kind of response at a low SPL. Raise the levels a bit and compression will certainly change that response. There is a big difference between the Monolith subs and the Rythmik subs; when you raise the levels on the Monolith subs, the response hardly changes, all the way out to the highest drive level. That is mandated by THX which is very strict on compression behavior. For THX certification, the response shape must hold at all drive levels.
As for the headroom versus your listening level, yes, there wouldn't be much of a difference if your sub had enough linear throw to satisfy your desired SPL. Although one advantage that a long linear throw offers is that the moving assembly of the driver stays in a very comfortable zone of operation versus a not-so-long Xmax driver that could be brushing up against distortion products as the motor approaches its limits even if it never surpasses those limits. What is more, the long-throw driver has the potential to last longer since it will never be stressed as hard.