I don't do many subreports, but there is more than one tool for you. the grand total box is on the main report, right? you can, technically, INNER JOIN 15 tables in one query if they're all related, but it's not practical. if so, you can query a total and reference the field, DONE.
if not, you might have to test other reference syntaxes. I do this kind of referencing often but you can try many kinds. in the control source of the box, you might try some of these:
Code:
=reports!subreport!totalbox + reports!subreport2!totalbox2, etc...
or you might just do it in code when the report is opened. you could loop them all, if the report names have a string in common, and the total boxes on all of the subs also have a string in common. It would also be much easier if these totals on all the subs have a common way to access their sources (tables or queries).
lastly, 15 subreports, IMO, is not at all close to practical. I would change the method as soon as possible, because anything structured like that is going to be a nightmare to maintain.