No, I don't have one and have never heard one. But I'm gonna defend it anyway...at least as far as marketing goes and at the risk of being called an Axiom brown nose....
I've hard a few people grumble that it's priced too close to the EP500, and, to be honest, that was my first thought as well. But in considering it a bit more, I think Axiom will fill a good nitch here.
You only need to spend a week or two on these forums to realize how many of us deal with serious WAF issues. For those people, the 400 might be a Godsend.
Don't think of it as a sub that's only $100 less than the more capable 500. Think of it instead as a sub that has almost the performance of a 500 in a WAF-approved package.... remaining the only option for many who can't put a large box in the corner of their living room or need a sub for a small room as touted.
In my smallish 13' square room, I think it might actually be a nice alternative to the 500 someday. It may not go quite as deep, but it will "blend in" much nicer than if I went with a 500. And I don't even have a WAF issue... !
If I can offer Axiom a suggestion, they really need a photograph of it next to the 500 for comparison purposes. A guy who can't get "approval" for a 350 or 500 would only have to point his browser to that photo to get permission for a 400....