Just out of sheer curiosity, how does one measure a soundstage?

Oh and btw, a friend of mine bought the Denon AVR-X6500 and liked it so much that he decided he wanted the same thing with more power so he was going to buy the 8500. I convinced him to let me "borrow" the 6500 for a bit to test out my new speakers (which arrive today according to FedEx). He still bought the 8500 but has until Jan 12th to return the 6500 I guess due to special holiday return policies.

Not sure if this will be a good idea or not though. I had/have the Denon 2300 and absolutely hated it which is why I bought the Sony STR-DN 1080 which really woke up my current speakers. The 2300 sounded fine but felt so low on power compared to the Sony it replaced (I think it was the 1050). So now I will either hate the Denon 6500 or love it and can't afford it lol.

M60
VP180
M2i
Sony STR-DN 1080