I am a full-time trained classical pianist who still regularly perform on various grand pianos so let me chip in on this discussion.
I think the bare minimum speaker I've heard that is decent to listen to piano was ATC SCM19 though it's slightly over 4K for a pair, but if you pressed me I guess below 4K I might settle with an Amphion Argon 1.
Currently using a Cabasse Riga setup and it's very lovely for piano and classical.