Edit:for the record, every person I have talked to on audio forums has said that polk makes great speakers for the price, but in terms of subwoofers, they really aren't good compared to other brands. So my recommendation of Polk Speakers speaks nothing about the subwoofers. Kind of odd that they make great speakers for the price, but don't do well at all subwoofer wise (in terms of audiophile perception of course).
For some reason the subwoofer realm lends itself to a number of very specialized vendors which I think is where the problem comes in.
As for the OP - some of this will depend on the type of music you listen to and how good the source material actually is. A lot of music is highly compressed into a small band of sound and they seem to particularly cut off the lower frequencies. If a lot of your music is the over engineered compressed shit you won't need a lot from your sub. If you are lucky enough that the artists\music you like is actually well mastered you will want to pay more attention (and maybe spend more $). I don't know what your constraints are but when I was looking the $400 range was miles beyond the abilities of the sub $250 range
There are a lot of options out there but some of my thoughts:
-Outlaw M8 ($250) Not the most powerful but some of the best responsiveness
-HSU STF-2 ($340) Obviously over your budget but a lot more power
-Klipsch RW-12d ($250) if you can still find it. Better power than the M8 I believe but maybe a tad less responsiveness. Probably the best uption for $250 but the least likely to actually be found
I know Energy and Emotiva have had good musical subs in the past but the Ultra line from Emotiva is apparently discontinued and I am not as familiar with Energy. I would still recommend looking into those companies though as they have a good track record.
My personal opinion would be to just suck it up and get something from Rythmik but you'd need to increase your budget