You should not have to change to a Proxy datasource to solve this. It is solvable purely in the Report Resource Designer program, I believe.
If the browse has one row per PO-line item, then the Detail section of the report design will render once per item, which is probably the problem. You should not put all the content in the detail section if you want some of the content to appear only once per PO..
Instead, you could create a Group by PO# (right click on the top-level report in the upper left part of Report Resource Designer, and choose Sotring and Grouping. Add a new group on the PO# field). Then in the Group Header section (which will fire once per unique PO#), put all the PO-header-related content, and in the regular Detail section (which always fires once per row) put only the PO-line-related content.
You are correct in saying that the detail section renders once per item when I want it to render once per PO. The browse also has one line per SKU against a PO as you said.
I have tried what you have suggested and grouped the data by PO number and it now renders the group-header and group-footer once per PO but the detail remains to render once per item so I am continuing to have the same issues of the detail section repeating itself x amount of time.
I have tried multiple settings within the sorting and grouping, however still can't fix the issue. I'll refer to the documentation on sorting and grouping but if you have any more suggestions would be kindly appreciated! :)