I recently replaced a pair of JL Audio F113 v1 with a single Gravis VI in our 24'x17'x9' game room to better match up with the Amati Anniversaries in this system, and am happy overall.
The single Gravis doesn't quite keep up with the pair of JLs in lowest bass extension and smoothness of frequency response (much more about 2 subs vs 1 than due to quality differences in the individual subs), but is more than sufficient for our basic 2 channel listening and movie watching. It is of course 500% better looking than the JLs, though I wish there was a bright "Red Violin" finish option to fully match the Anniversarios. It is also very convenient to switch between 2-channel and multi-channel without having to swap cables, only needing to adjust output level via the iOS app (with the 2-channel utilizing the speakon high level inputs and the MX123 coming in on the XLR LFE input).