1. I have the A500 driving the high end, and P500 driving the low end. This was the recommended installation set-up provided by staff where I purchased the gear, as well as in the Cambride manual. I have not tried the opposite.
2. I used the A500 alone for the first several months, before adding the P500 to my system. I don't recall there being a night or day "jaw-dropping to the floor' difference in sound, after adding the P500. The effects were more subtle, like a perceived increase in clarity/dynamic range (I say "perceived' for all the scientific method people on this board...I did not do blind comparisons!). I also added a sub to my system several months after adding the P500, which I found really filled out the bottom-end nicely.
3. I'm using 10' lengths of Kimber 4PR for the upper end, and Kimber 8PR for the lower.
Cheers and happy listening.
