The problem is crystal clear from your first photo (shown above)the center speaker is placed within a cabinet. All freestanding speakers, including the RC-64, are intended to be positioned with lots of free space around them (hence the term "freestanding"). This allows the sound to expand freely around the speaker. If you place it in a cabinet, the sound can get muffled and distorted. Granted, the RC-64 is a sealed-box designyour problem would be worse with a ported-box speaker, especially one with the port on the backbut still, I think the best solution is to get the center speaker out of that cabinet and onto the shelf above it.
From your first photo, that would mean raising the screen a few inches. But from your second photo (shown above), that might not be possible, given how close the screen is to the ceiling already.
You could put the center speaker on a tall stand on the floor, but that would be pretty ungainly in your otherwise elegant theater. (I wouldn't put it on a short stand on the floor, because it would be far from the screen in that case, and the front seats would block its sound from the seats behind them.) Another solution would be to get a slightly smaller screen that allowed enough room below it for the center speaker on the top of the equipment cabinet. If you watch a lot of movies, you might consider a 2.35:1 screen, which would give you plenty of room for the center speaker, though replacing your current screen would be expensive.
The easiest but least desirable solution is to EQ the center channel to compensate for the cabinet's effect on the sound. This might improve things somewhat, but not as much as placing the speaker in free space.
Update: Another option, mentioned in the comments below, is to add damping material inside the cabinet. THX recommends using a poly batting material similar to that found in pillows, filling the space behind the speaker so that it's moderately firm but not stiff. This will help reduce the resonances within the cabinet and clean up the sound from that speaker. You can also build a baffle around the front of the speaker to isolate the front from the interior of the cabinet.
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