...I like how my rig sounds right now with Audyssey room correction cut off at 500 Hz; and any attempts at further tweeks are more likely to screw things up.
You are probably well above your Schroeder Frequency with that 500 Hz cut off. Have you tried lowering it to 300 Hz or even 200 Hz?
I probably couldn't tell the difference but would rest easier knowing I was closer to the SF.