I have a PRS 600 as well and like the collections ok. But I must ask why no subcollections.
This is hardly rocket science as they even has subdirectories on CPM machines way back in the 70's.
For the price you pay the software is pretty basic and Sony does not seem interested in improving it.
I recently bought my sister a little ipod look alike for $29 CAD. It has 4 inch color touch screen, plays movies, audiobooks, has txt ebook support and all the subdirectories one could want.
Not the best for ebooks but the firmware/software is much less Mickey Mouse than Sony's