I'll bet it's the center one isn't it. Nope, not likely your imagination. As I had mentioned when designing the enclosure for you it's possible for the sub(s) closest to the port opening to move a little funny, even unload when the other subs don't. It's not something that can be modeled either, at least not at our level with easily to attain/afford software. There might be some FEA or other high end software that could but I'm not even certain of that. That's why that loading wall/brace was in there, to try to prevent that from happening. Apparently it's either not large enough or isn't in the right spot, damnit.