Don't.
What are you wanting out of these?
HT? Music?
If you are going to spend money on Subwoofers, I would strongly recommend that you determine your priorities. If you are chasing the Low C on a Pipe Organ (32' stop at ~16Hz) or the best movie experience, these will not do.
Also, how big is your room? Volume? LxWxH? Will these have the output to satisfy your needs?
Lot of questions to answer if you want to buy something you won;t regret and need to replace a few months down the line!
