Bob,
Lets get back to the simple stuff for a moment. How loud do you want it to be, and at what distance?
- If you want to feel the bass at the other end of the football pitch, you're going to need a large-format system which will cost more than your van, and won't fit in.
- If you want the music to be clearly audible, an 835 over an 828 will probably do it
- If you want it to be very much background music at the far end, you could probably get away with an 835 a side.
For what it's worth, I used 4x 15"s and 2x 2x10"+HF cabinets and was hitting 85dBC-slow at 260' distance, which worked out as 103dBC peaks while tickling the limiters. I was raising my voice to talk to the person next to me.
I'd expect the 835+828 combo to be somewhere around that level.
For the sort of thing you're doing, that might well be enough. I'd want more headroom for live music, but for playback firing across a rugby stadium, it was enough. Going down the length of the pitch might lose 6dB. For reference, +6dB sounds like "half as loud again", while requiring a double in the number of speakers.
Chris