You will gain both bass and highs with the Vandersteens. The TC-50 has great imaging, nice mids but nothing else. You will also hear great improvements by upgrading an old NAD. The Fisher should sound great assuming it works well.
My only qualm about the Vandersteens is that if you really like to rock, the tweeters may not take it that well.
My only qualm about the Vandersteens is that if you really like to rock, the tweeters may not take it that well.