What Instrument would you listen to?

Does anyone hear actually buy an entire rig preassembled? With a single instrument, you can get a feel for timbral accuracy, so its a good place to start, but clearly you have to move on to music that uses multiple instruments and voices to complete the evaluation.

But to answer your question. Piano first, followed in order by cello, violin, french horn, then soprano and tenor voices.
I have heard speakers that do all of these well but fail on piano. That is a deal breaker. On the other hand, I have never heard a speaker that gets piano right and fails on the rest. So the lynchpin is piano.

Theo, Beethoven Waldstein sonata, Andreas Schiff on ECM always goes with me on auditions.

Theo, Beethoven's Waldstein sonata is my favorite of the 32 sonatas he wrote. Also, it is a great piece for equipment audition. Very dynamic and engaging. You can listen to it 1000 times and not tire of it, which can be helpful when you are auditioning equipment.
I like to use good recordings of recent vintage for auditions. The Schiff account on ECM of this sonata is a pretty decent recording. Not sure if he used a Steinway or Bosendorfer grand for the Walstein. The Paul Lewis recording of this sonata is also probably very good, but I haven't heard it yet.

Almarg, they are out of print. Looks like they are going used for ~ $80-200.00. It also looks like some of his recordings are being reissued. I will keep an eye open for reissues.