IME for classical and especially to reproduce a 9' grand piano, it helps if the speakers energize the room in a manner similar to the piano. A dipole helps, but an omnidirectional gets it spooky right, of course with the other characteristics mentioned already--low level resolution, dynamics that can jump, truly full range, etc.
For example, the MBL Radialstrahler 101E Mk.II seems like a good candidate, especially when you consider this passage in
Michael Fremer's review:
With all of these recordings, the MBLs produced as believable a rendering of the sound of a solo piano as you're likely to hear from any speaker, whether the instrument was recorded in a reverberant church (Hegedüs, Artymiw) or the drier-sounding Henry Wood Hall in South London (Lill). I played them at the approximate SPLs I hear from my seat in the center of the 20th row of Avery Fisher Hall
The upper end Wilsons would also really do this well. I've heard orchestral and piano on the Maxx 3's, Alexandria X-2s, and XLFs. Fremer's review also compares the Radialstrahlers to his newly delivered Maxx 3's, which also did piano well in slightly different ways.
You may also want to consider Magneplanar 20.7s augmented by a pair of JL subwoofers to fully flesh out the slam and bottom octave. I've heard the 20.1s with a pair of Fathom F212s and they were also excellent. The lower price leaves more resources for top-rate cables and electronics, and the separate subs make it easier to place the 20.7s for best imaging and the subs for best bass. However, they can also be positioned right alongside the 20.7s for maximum phase coherence, which is how I heard them set up.
The large radiating surfaces of the Radialstrahlers and Maggie 20.7s definitely help capture the presentations of orchestras and grand pianos, but they also appropriately scale down for small acoustic ensembles.
