Thanks for your reply.
As I did in my previous answer, let me emphasize that aesthetics isn't my specialty, so I hope specialists will come forward to answer your questions. I'm not sure what to say about the idea that a musical work "conveys something grand" or "manifests a higher reality" than what's manifested by another musical work. So I'll leave that to others to address. But we might just compare Bach and Rihanna in terms of the harmonic and rhythmic complexity of their music; their inventiveness in developing a theme during the course of a piece; their skill in writing for various instruments; whether they incorporate enough surprise in a piece to maintain our interest yet not so much that the piece lacks integrity; and so on. Pop music almost always strikes me as very simple music -- it's often more "ear candy" than something having subtle flavors -- which may explain its mass appeal. Now, it's probably unfair to compare Rihanna to Bach, because by definition Bach's music has stood the test of time: we still listen to and perform it 275 years after he wrote it. Only time will tell if Rihanna's music enjoys the same longevity, but I wouldn't bet on it.
Most of all, though, I'd emphasize that the difficulty of settling aesthetic issues, or the lack of confidence we might feel in making some aesthetic judgments, needn't be a reason to regard aesthetic judgments as non-objective, i.e., as merely a matter of personal preference. Difficult issues and lack of confidence arise in many fields -- such as history or the sciences -- where we're not tempted to conclude that everything is a matter of opinion.