That James burial sub is a beast!  I've been pleased with output from the 12" Episode burial subs too.  Placing 1-2 of them around the yard with a 1000w crown amp would really improve the sound especially if there's no sub in place now.

Does the Sonance amp have DSP so you can apply a high-pass filter to the rock speakers to take the stress off of them and the Sonance amp?  I think it's important that aside from adding a sub that you also have a plan to block some of the bass going to the existing speakers.