Proac's slogan Perfectly Natural really does summarize their sound. They are more open and cleaner than the classically styled Harbeth, Spendor or Stirling models.
I run older Tablettes and they are my favourite minis. They offer the natural midrange of BBC school speakers but offer a more open top end without getting bright. They do prefer a larger room to a smaller one.
If you cannot find Proac, modern Spendor models are a great alternative.The Classic Spendor series sounds armer like the Harbeths.
All great speakers if you love the UK sound like I do.
The thing I love about Proac is that classical music sounds so natural but I can also put on some early Black Sabbath or Deep Purple and they are just as plausible. The only thing missing of course is really deep bass on the smaller models.