Lack in harmonic richness

On the other hand, I have never heard instruments "bloom" in the concert hall.

That's interesting as I was in Carnegie Hall in April and
even up 20 rows or so, the piano had tremendous bloom. I can only dream of having that sound in my room; but after a beer or two, it starts getting closer and closer.

The worst coloration is truncated harmonics. Ha! How is that for perspective?

Chris - I have heard a number of systems sound the way you describe: thin and unnatural. I hate that too. It could well be that you don't have enough bass. Adding a sub can do incredible things for the treble. I don't know how but it does and I have experienced it with several systems. In every case it fleshed out the mids and highs so finally vocals sounded like they came from humans instead of electronics.

Another is the source. I find many sources just don't have the right timbral balance which leads to poor sound. It could well be that is because of the recording quality but the end result is still the same. You have a fancy source but what you need is one that works with the rest of the system. If you have a McIntosh dealer near you, see if you can get an MCD301 on loan. That will tell you if it is your source or not.

Have you experimented much with speaker placement? I know those are big speakers that don't lend themselves to it but it is worth the effort - not to mention it's potentially free! Try changing the toe one way and then the other. Sometimes that is all it takes.

Arthur