Hi Robert,
Thank you for your interest in Syncfusion products.
To save an .xlsx file in xml format, select the version to 'Excel2007'.
To open that xml file in xls format, select the version to 'Excel97to2003'.
Please refer the attached sample and let me know if you still face any issues.
http://websamples.syncfusion.com/samples/XlsIO.Windows/F68689/main.htmRegards,
Jaya