That's a wide-open question!
I think you'll have more luck with suggestions as to the very few speakers in that price range that a pair of M-500t MkII's *won't* drive well. At least that will eliminate some options.
Are used speakers ok? Or are you only looking for new?
Do you want a speaker system that will give accurate, loud, very deep base, or do you have that well covered by your Sunfire sub?
Since you have VSA VR3's, do you prefer bipolar speakers?
What kinds of music do you listen to? At what typical volume? What is your listening room like? Can you post pics of it? What changes are you willing to make to it to accommodate different speakers?
You have nice gear already, and I'm not surprised that you love the way it sounds! But I'm puzzled by your statement that you often don't know what you're missing. How did you discover that you we're missing these things, how did you learn what they were, and what were they?