I have a Velo DD-15 in a 14x20 room. Ridiculous overkill, but eventually I'll have a bigger room for it. I had a great deal on it because I was in the audio store and they only had a private room model left. Knocked 1K off the price tag, and I've never had an issue with it.
Before I made my purchase I listened to a lot of different sub models, and finally went with the Velo, as I found it integrated really well with Paradigms. At the home theatre store I had them drag out their Signature 6's and spent an hour and a half listening to music with them and the velodyne before finally making my purchase. Mind you, Paradigm's own sub is a servo controlled unit as well, so I suppose thats why they marry so well.
I get the sense that you enjoy the sub, so I would definently take a look and see if you can get it reconed. Failing that, I would see if you can trial some subs with your current setup before settling. You may find that the sound from ported isn't to your taste anymore, 15 years of the same style of sound can grow on you.