The thing you have to remeber is that under normal listening conditions you will most likely notice little to no difference between those SVS subs (once calibrated). Sorry guys but it is true, I've been there many times with SVS (among others). I mean honestly, when you are talking about tuning at 16 or 20 hz to gain a few more spl's, you lead a boring life of churchy, organ music. the majority of musical bass is 30-35hz and up. Then there is home theater. once again, most of the movie bass is 30HZ and up. Yeah yeah I know WOTW has a great scene (or two) that will cause your spleen to rupture b/c the bass is so deep, but how many times can those geeks honestly sit there and watch that scene over and over and over when there is so much great music out there to explore? If you do a lot of "critical" listening, the differences may show, but 80-90% of the time you wouldnt be able to tell. That being said, if you can afford it, you certainly wont regret getting the more expensive SVS.
