Searching for the most "accurate" speaker (below 15K)

15k is a generous budget. If your room is 10x14 or smaller suguest a used pair of Wilson Sopha 3's, (and invest difference in S&P spiders), if larger room size then again used Wilson, but larger Sasha's. Both are very accurate, World Class. Enjoy!

Nagra Classic amp and Wilson Benesch Discovery 2, in a 20 square-M room.
This will be my choice and an option for your consideration

As @willemj has pointed out, this is a smallish room. I’m surprised by the number of recommendations for speakers that, though terrific, are likely not the best fit for the room.

@vermeer Can you tell us more about your room? Is your room dedicated for audio or is it a mixed use room? Does it open to other rooms / spaces? Room construction material and height (since you are in Portugal, if it is older construction it is likely different than what is typical in the U.S. and Canada).

Vermeer,

have you considered Wilson Audio?  I know the secondhand market in Portugal is relatively small, But if you can get a pair of Sophia 3, Watt Puppy 8, or even Sasha 1 used, you will get some amazing resolution and detail.  Can your nagra amp go to 4 ohms?

what part of Portugal are you in? Are you dealing with Imacoustica?

At under 15k the pair why not go for the best (IMO) and pop for a pair of Magnepan 20.7 's.  Pretty hard to beat for the money if you have the room.