Here I'll sum it up for you. You're a JL nut hugging want to be. Enough said. This is why I hate forums. You can't give anyone any advice without some idiot telling you're wrong. It's ADVICE on a brand of sub. How am I wrong? He asked about sundown. I offered a few other brands. Get a life.
lol, umadbro?
And, yes, the OP put "Sundown" in there, but that doesn't mean one should toss-out a laundry list of names---- as you did, at least twice---- when he also indicated he wanted SQ. (Most who remark about wanting SQ are actually more about SQL, and not looking to invest thousand$ into Mac equipment.)
You come beebopping along and regurgitate a bunch of names you've read at some point, with only a few known for SQ....kinda like tossing mud against the wall to see what sticks.
With the OP's budget, he has plenty of choices. Unfortunately, tho, he might think you know what you're talking about, and go for a DD sub or a DC Level 4 when his budget and his wants would allow him many better choices.
Now, granted, a subwoofer is not going to be the heart of an SQ system and installation is a key point, BUT, better to point him towards the lines of Morel, Dynaudio, etc.
More in the middle range, maybe suggest an IDQ, Dayton HO, Fi Q, and, yes, even the JL W6 that folks love to hate.
(Just to toss out another sweet little SQL sub that nobody gives much credit to: Infinity Reference 1060w / 1260w. Nice noise on the super cheap!)
In the end, I say that you shouldn't be offering advice on a topic and then be crying because someone tells you your "advice" suxxx when it does.