I wish I could help, but I have never used an RSS feed! Hopefully someone will have a good suggestion. If so, let us know how it goes and if I should try it.
I've used and enjoyed both ReadKit and Reeder (my current one). I've never had any difficulties with marking items as read. I do not, however, use the Logos RSS feeds—it would be too overwhelming for me.
I use ReadKit and like it. Like Integ I don't use it for the Logos forums as (as far as I can see) you get everything from all the forums in the RSS feed and it gets too overwhelping.
I use DevonThink for this, as I make my notes in this program too.
As I have used many Bible programs, I am happy to have my notes in a separate program :-)
Hans
Maybe I'm missing something ... it's happened before.
Podcasts are RSS feeds and iTunes reads/subscribes to podcasts. Maybe iTunes is limited in some way?
https://rss.itunes.apple.com/
http://www.manythings.org/podcasts/how_to.html
http://en.wikipedia.org/wiki/RSS
http://itunes.rts.edu/rss.html (sample)
Is there a specific issue or something I'm not aware of with the RSS feed you want to subscribe? Maybe I'm thinking about audio and you have a text RSS feed. Does Mail handle that?
https://discussions.apple.com/thread/2690828