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Well, maybe I spoke too soon in that post. When you first make a signal chain swap, it's easy to attribute all the new things you notice to higher resolution in the change.
Since I posted in favor of the built-in phono stage I've gone back to the Cambridge 640P outboard phono stage. Although the Cambridge 640P is relatively inexpensive, I think you have to spend $600 for a Musical Surroundings Phonomena to get appreciably better.
What I decided overall is that the built-in Onkyo is pretty good, and is particularly good at resolving inner details such as subtle vocal backings and the microdynamics of solo voice, cello, string quartets, etc. In the end, however, I noticed I was missing the slam, dynamic contrasts, and rhythmic drive that the Cambridge does so well. I also found that the Cambridge also resolves the subtleties well; they're just not as obvious because its ability with transients can obscure some of them, but they're still there.
My verdict: The built-in phono stage in the Onkyo A-9555 is a nice little phono stage that will bring out the inner detail and subtleties, but at some point you'll probably wonder where the transients, rhythm, and drive are and want to insert at least a Cambridge 640P or Musical Fidelity V-LPS in the chain.