I am a EP350 owner for about 1/2 year and have been drooling over the higher end Axiom sub. It seems like there is not much discussion about the EP400. Just wondering why. Unfavourable price point?

Unfavorable price point and lack of total output, it is strictly for very small areas or music only systems, which is what it was designed for.

I demoed one for awhile and found it to be a great sub for a 2.1 system. It just doesn't have a large enough driver to add rumble and shake in a HT environment and the price makes it very prohibitive, to me anyway.

So either stay put or go all the way to EP600?

I have yet to hear the EP500 but I would suspect the EP500 and 600 are very similar so you could get the 500 as the next step up unless you have a rather large room ie >4000cf.

If you want to stick with Axiom, and watch mostly TV/movies, I would consider the EP500 (I own one). It won't disappoint when it comes to HT AND music. The EP400, from what I've read and heard, is more for smaller rooms for music (as jakewash said). However, at the EP500's price point, there is no reason not to consider offerings from other companies, especially if value is a high concern. Especially also, IMO, if you mostly watch TV/movies.

I have not heard the EP600, but again from what I've read on this forum from others that have compared them, the differences between the EP500 and EP600 are subtle. The EP600 is said to perform better with music and go a little lower.

I've heard both the 500 and 600 and both give a very impressive rumble. Both were in moderate sized rooms (< 2500 cubic ft). I did not hear them side by side, but my impression from the 600 I own is that it goes a bit deeper and hits a bit harder. I think you'd definitely notice the immediate difference with either compared to what you have now.