Marc,
If you want something substantially better than your Zus, then Harbeths are not it. They'll be pretty similar to your Zus, down to their obvious colorations and deficiencies. And they're inefficient to boot.
You're in Europe, so take advantage of that, and look for a used Avantgarde. I'm seeing Trios for 18k euro, and Duos for 11-12k. You will NOT find speakers more suitable to your musical taste than those, for these prices. And you'll still have all the flexibility in the world to choose your amps (or you might as well just keep the NATs).
Then, sell the Zus and invest some of that money with a dealer/acoustician, and have the Avantgardes properly installed/dialed to your room.
cheers,
Alex