I think it is great that you are taking this approach to evaluating the subs and am looking forward to your observations.
Certainly Umik and REW are the best way to properly compare.
One of the most important things I have learned when making subjective A-B comparisons is to set up instant level-matched switching between A & B.
Get one of these ($17) and hook the sub out from your amp to the R channel output (that's right, we're running it backwards, it doesn't know the difference, it is just a dumb switch between two sets of contacts), then hook the RSL to the R channel Input 1 and the Rythmik to the R-channel input 2.
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Use the level controls on the subs to match the levels - expect to take some time doing this as our hearing is not great at the lower frequencies and the subs will have different frequency characteristics. If possible, use white/pink noise to do this.
I don't know if you normally use a roomEQ system such as Audyssey, but unless you have two equivalent AVR's to set up (which would not require the switch box), I'd consider it a reasonable assumption that whichever sub sounds better before roomEQ will sound better after EQ and disable any type of EQ or tone controls at the AVR.
It will still be a sloppy test from a scientific standpoint, but I think it is about as good as can be expected without making a major project of it!
Additional notes:
1) Check you CD/DVD player for an A-B loop option that allows you to specify point A and point B for your player to loop between. It is a feature on many units that you would never use. I forgot mine did it until I saw the button on my remote marked "A-B". I find this very useful as you can find (for example) a 3 second passage that you think sounds different between the subs and have it repeat "forever" (as required) as you verify the nature of the differences you hear switching subs over and over.
2) Understand that F3 at 24Hz was probably based on an anechoic or quasi-anechoic measurement (IOW, the response in an anechoic chamber where there are no sound wave reflections). For low frequencies, it is very likely that the influence of reflections and in-room resonances will be significant and that is generally likely to give you back the 3 to 6 dB you are down in the low 20Hz range. Just be aware that the published FR is not that good of a predictor of your in-room response.
3) Do your best to tune either sub before the comparison in the room using each sub's control options. If you have not dome the sub crawl, it is useful not only in determining the best position of the sub, but also giving you a good sense of what you are listening for and how different locations result in different character of sound quality! Obviously you cannot locate both subs in that special "sweet spot", so the best option is to locate them as close as possible to sharing it. Once you reach your conclusions, swap the subs and revisit the bits of music where you heard decisive differences (take good notes) and make sure that the characteristic is attributable to the sub and not the position in the room!
4) Run the initial setup routine with both subs to see how much delay/distance gets assigned to each. I don't know if these two have DSP systems or not, but if so, the conversion to the digital domain and back will delay the response. For bass, I don't think we are very astute at detecting when the sound starts with precision (don't believe me, unplug your speakers and only listen to your sub playing some of your favorite tunes)! Usually the attack of that bass note or dinosaur stomp is actually defined by higher frequency constituents/harmonics (like the sound from the brass string "snap/slap" when an impactful note is played on the bass guitar - again, listen to the sub by itself if you are not sure you believe what I am saying)! But it is reassuring if both subs measure within a few feet for distance.
LOL, I hope you can appreciate that I have tried to keep it to reasonably practical measures to give you an optimal result from a less than optimal pretense (subjective evaluation)! Whether or not you follow my suggestions to the hilt, having the awareness of these aspects should be useful.