The 1.5 cu ft box takes in driver displacement, Anything from 1.5 to 2.0 will work. Really in your case though since you are limited in power, I would build closer to 2cu ft per driver. I have 8 on a 6000dsp in around 1.9cu ft and it works very well. Basically lets the driver move more with less power if you build a slightly bigger box.
I started with 4 behind me and then moved the 4 up front and personally I liked the 4 up front more. I dont need to feel a car door shut and get kicked in the back. After that I bought 4 more since I had the 6000dsp anyways and now have 8 up front and love it. I still get tactile response, but my personal preference is I like them more up front, 4 in each front corner.
Putting 4 behind you will make the 4 up front kind of worthless since youll be getting such a pounding from the 4 behind you. A lot of people who do 4 behind them arent running them with a high x-over to avoid localization. Upfront you can run them to 100-120 and it blends with the mains a lot better (I think). Definitely feel free to experiment though.
- 6 BA CR6 array center channel, QSC AD-S82 L/R, 4 jbl 8330a surr , 8 jbl 12" subs w/Inuke 6000. JVC rs420, Denon x4000, Sony x800 -