I had the same issue with "undefined" actually showing as the text in my webcombo.  I changed to MultipleColumns and now the drop down does nothing.  This is integrated in a webgrid.  Do you have a sample where the webcombo is integrated into the webgrid and then depending on what is chosen in a cell, the rows in the webcombo are different?

I have tried as you have suggested but the JS event is never fired.  From reading the help file it appears that event should be fired on sorts, groupings, refresh, etc, is that right?  Because it never fires at all.  There must be some other setting that I am missing.  What else needs to be set?