Hi Clarky,
I'd suggest trying all the DI's as they will likely all sound a little different (power amp will be Pre EQ, Pre amp will likely give you the option, and the sansamp has it's own flavour). You may find one of the DI's has a natural sound that better suits you so run with that. Have a play in the practise room through the PA.
As for the board, as it's relatively small it shouldnt make too much difference. I'd run the Polytune last as it will double as a mute switch. If you run I first, it may mute the bass, but not any additional noise the compressor or sansamp put in line. I'd suggest going into the sansamp 1st as that's going to boost/EQ a signal, which you can them compress when compression is required. Again I'd suggest the golden rule though... Try it both ways and see which sounds best to you!
Shep