Ultimately you want to pick a speaker that is going to deliver more of what you love about Thiel. I would specifically want to know what your preferences are.
That said, Kef, Focal, Marten, and Vivid are common upgrades with Vivid delivering one of the more pleasing sound profiles. No secret I am a Vivid dealer and am a big advocate. I personally am a fan and it is one of a few brands I listen too at home.
That being said, the new Kef Metas are outstanding and one that you would be wise to look at. Marten is also brilliant and one of the better sounding speakers I have heard. I am not a Kef or Marten dealer.