The first error with your feed is very strange, something is adding to text to your feed where it does not belong. You will need to disable plugins one by one until the problem goes away to determine what is causing it. It could even be your theme, so try the default twenty twelve theme if you suspect it. This needs to be fixed, this makes the feed nearly impossible to parse for most services including iTunes.
The second error, your media URLs have spaces. When you use PowerPress and enter such a URL into the media URL field, you should get a warning. Rename all your media files replacing spaces with a character such as dash or underscore then re-enter all your media URLs you change into PowerPress, the warnings should go away from PowerPress and the error in feedvalidator will go away as well.