Best Speakers with a DNA-1?

Steve McCormack still upgrades the McCormack line. In fact, if you can find a broken DNA, he will upgrade it to your specs at a very reasonable price.
He made my DNA-1 monoblocks last year. They sound superb.

Regarding the Vandies, I suggest contacting John Rutan (audioconnection) and getting his advice.
I started with the 3a sigs, added a pair of 2w subs, and then bought ctsooner's Treo's. They were a significant upgrade.
If I were to do it again, I might just have gotten a pair of 1ci's and a pair of 2w or 2wq subs-which should be around your budget. The 1ci's are a lot easier to position (esp. in small rooms) than the 3a sigs (which have huge Sound Anchor bases). And, you should get sound comparable to the 3a sigs, possibly better-the Vandy subs are really amazing).
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Also remember that buying the most recent issue of the Vandersteen line is best, as Mr. V. constantly upgrades models as they are produced.
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Vandys are great as are ProAc and Joseph Audio.  Depends on your particular tastes and what what sound characteristics are most important to you.  I'm sure they'd all sound fantastic with the DNA1.  Great amp. 

@gdnrbob ++1 Two great posts. My DNA-1 is arriving at SMc Audio early this week for a complete rebuild. I bought my amp back in 1997 so the motherboard is probably about shot as per SMc Audio’s experience with DNA-1’s of my vintage.

Steve, Patrick etc. with SMc Audio do great work. They put a lot of pride into their work. And yes, their price for all the work they are doing seems very reasonable if not a downright bargain to me.

My DNA-1 is is driving Vandersteen Treo CT. Any Vandys will yield great music. Don’t forget what gdnrbob said about getting a recent production model as Mr. Vandersteen is regularly upgrading his designs.

Good luck!

Funny as folks get mad at me when I tell them to try adn purchase his newest models as he really does upgrade constantly.  It's also nice to be able to get some of the upgrades after you have owned your speakers for a few years for a nominal fee.  

Didn't even know Steve still upgrades and is building.  He is one of the forgotten builders I think.  I loved his amps as well as Brown Electronics Lab or BEL amps and a few others.  They would have been some of the first builders who would probably have sold direct back in the day if they really had that.