Hi Lee,
I would suggest you to set the ConnectionString and the RecordSource properties for the DataSource of the subreport in the OnFormat event of the section containing the subreport field in the main report. Here is the VBScript that can be used to accomplish the same:
Field1.subreport.datasource.connectionstring=Report.datasource.connectionstring Field1.subreport.datasource.recordsource=SQLSTR.Text
Also, please find a simple report XML implementing the same attached and it should work as per the requirements you are currently looking for.
Thanks and Regards,
Pragati Kaushik