Did you find anything? I went with Viking last summer. We were quite pleased. That said, it seemed to be more about the eating and drinking than the places that were visited. We had shore excursions everywhere, but they tended to be short. Excursion leaders, all locals, were very well versed in their subjects. Food was terrific, though,and so was the ship. Ages of participants were older than expected. Many guests did not even go on the excursions?!

Late to this party, but cruise critic has an entire forum just for river cruising.

http://boards.cruisecritic.com/forumdisplay.php?s=032e61c9dbec15ab97084b622f935801&f=191

Best place for info on cruises and cruiselines.