Upgrading my front speakers

Hi !

After several years with a 7.1 Focal Sib Evo kit, I decided to upgrade the front speakers (for a start). I want bookshelves, front airhole and something musical but not too much demonstrative. I'm an mixing and mastering engineer and I'm used to listen music with monitor speakers very flat. My budget is about 500€ and I'm absolutely open to second hand market.

What do you think?