I'm doing that in code-behind but the reportViewer control can be placed at design time with no issues "even without scriptManager" [at least in case of telerik reports but should be also with MS one].
please notice the use of Content Tag instead of Items.
<ext:Panel ID="pnlReportView" runat="server" Title="Report Center">
<telerik:ReportViewer ID="ReportViewer1" runat="server" Width="100%" Height="700">
then in code-behind assign the data source, bind and finally use the segment above .Update().
nothing more i can tell at this stage...try it out and do little research!
this.ReportViewer1.Report = this.GetReportObj();