Try to avoid brick and mortar subwoofers like velodyne if cost is an object.
Look into offerings by HSU, Emotiva, Danley, JTR, Epik, Rythmik, SVS, Funkywaves, and also consider building your own. You will get much better performance for what you pay.
On that note you have to consider your goals (output, extension, and sound quality), space constraints, and room into the decision.