Thank you for your feedback.
There shouldn't be problem using RadSlider "Prometheus" with an AJAX panel. You can use whatever version you prefer. For your convenience I prepared a sample scenario with a RadSlider "Prometheus", a DropDownList and two RadAJAXPanels. The example demonstrates changing the value of the slider with the selected one form the DropDownList. Please, find attached the sample application.
If you need further assistance, please, contact us again.
the Telerik team