I currently have a DAC9 + 2 x STA9 and they have done great with every speaker i have hooked them up to.
I think the UniFi recommendation is a great one. Would certainly put it on the list. The one complaint i have about it was the vinyl wrap finish, which i just didn't like. The Euro version is supposed to be a nice glossy finish at a slightly higher price, but if you are in the US i believe they were going to make it available here as well. maybe John can answer that.
The KEF LS50's can easily be had used for about 1k. Personally I didn't like them but many, many others would disagree with me on that.
Both of the above are lower on the efficiency side. A second STA 9 in the future would be something to consider if you listen loud, in a big room, or just find you want more power.
Other speakers i would look at would be an older Paradigm Signature S2 (used if you can find it), Golden ear Triton 7 or Aon 3, all of which should run fine on a the single STA-9. Probably won't find many on the used market but a pair of Fritz Carbon 7s might also be around your price. Fritz is a really nice guy and makes some pretty good speakers. Would be worth contacting to see if he has something in your budget.