I use two but I have used a distributed bass array of four placed asymmetrically within the room which eliminated all the rooms standing wave bass modes and provided immediate sense of timing or room loading.
I've since manage a similar but not as total effect with two bass mode carefully positioned subs.
JL Audio makes a potent high quality product. Personally I found their F113 automatic / frequency only optimization lacking the flexibility of added manual adjustability. This Youtube shows the Quality.
https://www.youtube.com/watch?v=GQ10cQ9O2L8
FYI, since REL are -6dB "Sub-Bass Systems" their frequency rolloff begins so early they usually don't excite a rooms standing wave bass modes allowing for greater flexibility of positioning at the cost of actual subwoofer extra low frequency presentation. Claims of superior musicality are purely subjective.