I've read the various posts on what balanced vs. unbalanced means. My question is, how much of a sonic difference does it actually make? Does it have any dependence on speakers, etc? Will I notice a huge difference between the 7200 and 7700?

For reference, I have:
Mains: Polk RTi12
Center: Polk CSi5
Surrounds: Polk FXi3
Sub: Non for the moment

I haven't heard the 7700, but I substituted a 7500 (using balanced connections) for a 750 (unbalanced, since it lacked balanced inputs) and found some subtle differences (as outlined in my 7500 review ). I'd expect the differences to be even smaller between the 7200 and 7700, as the 7200's design is a newer design than the 750. Definitely not "huge" differences, but there could be some subtle differences. The front end (receiver or pre/pro, ...) is likely to make a more noticeable different.