Marantz PM 8003 v Rotel or NAD sperates

The first thing I would consider, is the companies you list, are all known for making integrateds. They do make separates, but I don't think they would a very big step up in terms of sound quality. I think the best thing you can do, is to go audition some new gear, but don't pre judge it based on the design. Just pick the one that sounds the best, regardless of design.

When I owned the NAD C162/272 combo, I felt it sounded much better than the top NAD integrated of the time the C372, and well worth the additional cost.

though if you prefer Integrateds there are many very good ones in that price range.