I was in your EXACT same position about a month ago...same subs, same price level (~$400). I also considered the Adire Rava and the Rocket ULW-10. The price on the rocket was intro at $399...it's since gone up to $499 so that one's probably out for your budget.
My two strongest front runners were the SVS and the Hsu. Now, obviously I can't tell you which one is better...actually, I don't think anyone can. But, at $30 more than the Hsu, I thought the SVS wasn't really that much of a price jump. What I can tell you is that I've never looked back. Not one inkling of wondering "what if?" This sub has performed beyond my expectations and I sit around wondering why i waited so long to get a real sub (I was using an old HIB Sony 8" 100W sub...that thing barely put out anything). What I can tell you is that I based my decision on reviews, the consumer demand for the SVS (it was pre-ordered through the roof!...everyone wanted one before they were even shipping), and the quality of build put into. That's no knock on the Hsu...they look very, very well built as well. But there was just something about the SVS.
So, buy what you will because our choices are great. Also, if you're not happy, they are returnable. SVS gives you 45 days....I'm not sure of the return policy on the Hsu but I'm pretty sure they have one...I think they really want their customers to be happy. Although, I don't know about the DIY kit...seems like once you build it, it may not be returnable...but I could very well be wrong. My vote would obviously be for the SVS, but I'm sure a Hsu owner would argue for the Hsu. Ultimately, it's what you're happy with...luckily, you've started in a good area.