/* No need to put the .rdl ext after the report name */
execInfo = reportService.LoadReport("/PathToRDL/RDL_Report", historyID);
/* Ensure you have all the parameters set required by the report */
reportService.SetExecutionParameters(Parameters, "en-us");
/* EXCELOPENXML will produce a XLSX */
var auditReport = reportService.Render("EXCELOPENXML", devInfo, out extension, out encoding, out mimeType, out warnings, out streamIDs);