Redline series is NOT but DD sells a premium product and I'd expect your local retailer/installer will take care of you assuming you've vetted that shop as being good.
The Skar subwoofers I've seen around the area are nothing special apart from trying to look like Sundown. They're not great performers for their price even in the basket of bargain priced Chinese cookie cutter gear, and the owner has done some shady stuff in the past which is honestly the reason for much of the hate he gets around here.
That all said, instead of getting two 10" DD600s I'd suggest getting 1 700 12" or whatever. Go down to one sub, the next size up and into the 700 (cast basket) series. I've seen the 700s in action and they're really good. Make sure your shop builds the box to DD spec and they will deliver the goods. Their amps have a solid track record of being very reliable as well.
Don't listen to the guys telling you whatever you're looking to spend is too much. If you don't have the time, ability, or inclination to DIY install and don't want to worry about what to do if something goes wrong there's a lot of value in a brick and mortar retailer/installer and I can assure you it isn't cheap to keep the lights on in a commercial building and pay for good help. Obviously not all shops are great but the established ones are usually worth their asking price.