For the price of those subs, I'd go with Audioque, Digital Designs' sister company.
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I have never heard of them and based on the searches I have done not many people have heard of them either.
That's not to say they aren't good, but Diamond Audio D6s are great subs and DA is obviously a very well known brand.
I'm always up to try new setups though I would be interested in hearing them sometime.
I installed two of their SD2s in my brothers Tiburon and loved em. Here's some SPL vids of them, just to show the air and such that they move.
Realm of Excursion
I myself only run DD subs. Audioque is made by the same manufacturers with the same materials.
The only problem I have with not buying from a Diamond dealer is the first thing on their site... DO NOT BUY DIAMOND AUDIO PRODUCTS ONLINE!
Maybe i'm wrong but usually when there is a disclaimer like that there was a reason for them putting it up...i.e. counterfit product, not new product, ect.
If it bothers you that much then don't do it then man. I bought my d6 12" subs online and couldn't have been happier.
It's kind of like the "Do not remove" sticker on your mattress. People remove them all the time. They just put it there for liability reasons.
Ok that wasn't a very good example LOL, but bottom line, if it worries you don't buy them online.