I think the Martin Logan Dynamo 700 is the new version of the old Dynamo, which was a good little sub. I think it played low, but not necessarily loud. The Dynamo also has the option of the wireless swt. It's tiny and light.
The Rythmik is a little more than budgeted, and may not be in stock, but worth looking into -http://www.ascendacoustics.com/pages/products/subs/f12se.html
I've never heard the Emotiva Subs, but from what I've read in reviews they made trade offs for more volume vs. lower frequencies. Which would make them better for home theater or for filling a larger space. Not sure if the new X-Ref series is the same.
Don't know anything about the REL other than what i just read on the site. Wish REL would publish better specs. It sure is tiny, and comes in nice finishes, definately high WAF.