Dear @james633 : " I don’t see anyone using JTR stuff for music.."
Well the main reason is that JTR subs are designed for HT in specific and not for MUSIC systems.
In HT THD is not to important and even that the enty level RS1 measured more or less " fine " its THD goes to high but your JL are not way better in that regards and certainly not only in the RS1 price range but with out the " impact " in the low bass that has the RS1.
Btw, it looks a lot to the Velodyne 18"/15" that are higher in price but more important is that Velodyne's THD goes at 0.5% thank's that the design checks in real time over 16K times each second the woofer excursions.
In the other side: " I would like to try 4 at some point.."
Of course you can but at one specific seat position we need only 2 subwoofers and Harman showed 30+ years ago why even that in its study said the optimal are 4 for the " room space " not one and only seat position as with dedicated MUSIC system .
R.