Please consider that without a data set you are speaking gibberish/greek/something! Instead of focusing on the specifics of your data set, because you have not shared a workbook or data this sounds like a possible usage of a Level of Detail Calculation (LOD). The LOD allows you to look at more than one level of detail in the same view. By default, and much to your chagrin the level of detail is based on the things you use on your columns, rows, filters, pages, etc shelves. But you use a function called EXCLUDE. It will allow you to remove effects of Substage.
Here is a link that will help - with more data others can be of assistance or you can internalize this blog and rock your own issues! Enjoy!
thanks for your reply, and I appreciate the usefulness of supplying data or a workbook! I'll make sure I do that next time.
Incidentally, through (a lot of) trial and error, this seems to be working as I wanted now. I ended up changing 'Substage' on my Columns from a dimension to an attribute, and 'Income trend' on the Colour shelf to an attribute. I've never quite got my head around Attributes but to my understanding Tableau is no longer aggregating on those dimensions and so now is able to display the Products as I wanted.