Any speaker in a cabinet wil be affected, sealed versions like the VP series are affected minimally especially when the leading edge of the speaker is protruding out slightly from the face of the cabinet as has been mentioned previously.
The VP100 is a nice speaker, it sounds very similar to the M22 just wiht a little less bass tone. I think for your application they would be a grreat choice.
Yes the center can be lower or higher than the mains, this is how many people run their systems and for the most part it sounds and works great.
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