That's perfectly normal, we (mate and I) love listening at insane levels at his place and regularly dial it up to around 0 - +6. I would attribute this to the sheer quality of the 8000ap, the extreme low noise floor and transparency in sound fools us into thinking it isn't quite as loud until something happens and you're like, damn!! It really is an amazing piece.
With regards pre-outs, the P7 follows THX standard of 2V I believe, not sure about the 8000p but you could see if they're matched from the manual and also whether it's adjustable. It shouldn't matter too much though.