There is no correct phase setting, its what ever sounds good.
Volume should be at the point when it blends with the mains, So 50% might or might not be right.
For 300, thats a steal. Its the same sub as the SPL800R but without the R(remote and auto EQ)
Sometimes I find that you don't need the bass turned up very much for movies. On the other hand, music needs to be a little louder. Its because of the DVD's and the way they are recorded.
Try messing around with the phase, and volume and see what you come up with.
SheepStar