Welcome from a fellow 708P owner. Very good speaker!
Have you read Floyd Toole's "Sound Reproduction" (have read the book), anything from Earl Geddes (been to his place), or visited Duke LeJeune's AudioKinesis mini-circle here at Audio Circle (AC, he is a very helpful guy). They all point to the reasons/advantages of running 3 or 4 subs for in-room performance. While certainly not a brown sound or bass-head kind of guy, I do believe that one of the basic requirements for true hi-fidelity is 10 octave (20 - 20,000 Hz) coverage.
I run a 2.3 system off a PS Audio DirectStream Junior (DSJ) DAC/preamp/streamer that is fed by a MacBook Air/iTunes/10ft USB cable in a mid-field setup in an 8ft x 13ft x 21ft well treated/insulated/dedicated room. DSJ has one pair of RCA and one pair of XLR outputs. My (1) 10 inch CSS XBL^2 sealed sub is connected to both RCA outputs. You can find CSS in the industry introductions here at AC. The (2) PreSonus Tremblor T10 subs are 10 inch ported professional subs and are connected to the DSJ XLR outputs. Like many professional subs they have lots of input/output options. So both low pass and high pass filters are set at 80 Hz and XLR outputted from each of the subs to one of the 708Ps. The T10s are located near front left corner and 4 ft from front right corner along side wall. CSS sub is located roughly 2/3rds the way back along left side wall.
The subs only cost $1100 total, the CSS being used. Frankly other than adjusting the volume I've not done much more with them as I'm satisfied with the sound. Initially felt that the ported T10 subs were inferior to sealed subs (a bit of hangover) but I've either acclimated or they've broken in. Note that I'm still a big fan of transmission line (TL) bass, but TL subs are impossible to find. My goal is not ultimate sound at any price, if that's your goal I'm sure expensive sealed subs would do the job (assuming you have the right room).
