So while changing the sort order to take into account title articles, in Tellico 1.2.7, I broke the secondary and tertiary sorting, which is useful for series and series number. I don’t expect to make a 1.2.8 release quite yet, so here’s a patch.
Or grab the SVN revision.
You could also just define another field that combines the series name and series number. The field type is Dependent and you put %{series} %{series_num} in the field description. That combines the two values and lets you sort quickly.