What about HSU? They make one for like $300-$350 that seems to get excellent reviews. And I have heard that Energy makes a nice one. I'm thinking about those two, probably Energy, since I'm looking for something very-musical/engaging (if that's possible in a sub). Something with good PRAT skills that gets your toes a' tsppin'.
I'm actually looking for a sub myself, something that I can use with both my W90's AND my Denton reissues (But separately, of course... I don't run both pairs simultaneously). My biggest concern along with budget is making sure the sub matches up well-enough with the W90's. The Daytons with their small woofers should mate quite-well with most subs, I would think. But the W90's have much larger 12" woofers and were made 50 years earlier than the Dentons, so I'm trying to find a sub that can maybe meet the two models somewhere in the middle. Not really sure though, as this will be my first real sub, which is why I've been a patient, cautious shopper on this one. My main concern is the W90's. If I can find the right sub for those, it should be quite a thing.
But like the OP, I'm on a budget of around the $300 range, so I've considered going with something used, but I really don't know subwoofers that well. I ran a Soundstream SPL15 that I really-liked in my car system (and a couple other models I used back in those days), but I really had a lot to learn back in those days in terms of application as well as what kind of sound I really wanted (I was still finding my audio self, so to speak). My gear was good. I just didn't know how to make it really shine.