Well, it's finally figured out and resolved.
The issue had nothing to do with the enclosure tags - they were always there.
The problem is that we installed Wishlist Member onto the site for an upcoming membership migration and Wishlist screwed up the PowerPress feed by adding some private key on it that broke it in iTunes.
Bottom line is we'll install wishlist on a sub-domain instead to avoid this grief. There appears to be a fix to make this work with both Wishlist and PowerPress - but after this ordeal and drama, we've just decided to keep the membership on a completely different sub-domain and "keep it simple"...
Thanks for the assistance Roger...