I didn’t know amember would block rss feeds…odd that it would do that. Have you looked at Pippin’s Plugins for their restrict content membership plugin?
However, with regards to your issue at hand, I’m thinking that perhaps the amember is causing a conflict, which is what you are probably correct on that as a possibility. If you disable that plugin, does the CSS classes work? That might confirm it.
Still sounds like it probably is amember. If it is, check out Pippin’s plugin Restrict Content Pro, but send them a brief detail of what you are currently experiencing and wondering if this would happen with their plugin. It is a paid plugin, so you will want to make sure before paying them. They have very good ratings and reviews. But first see if disabling your amember is the culprit.