Thoughts on the most difficult instruments for speakers to reproduce?

Another +vote for a piano. It is a percussive and melodic instrument.
Happy Listening!

I can think of a few things.

1 - Microphone placement - Audience, stage or in the case? This all affects what is in the recording to begin with. 

2 - Bass - This is why so many feel subwoofers can add so much to music. 

3 - Radiating patterns. Piano's radiate spherically, with some direction given by the main lid. Some sounds are mechanical and coming right off the floor. 

4 - Room acoustics. I heard the Magico S1MkII in the Magico showroom - they did great except in the bottom registers. A little too chesty due to the speaker tuning. However the rest of the piano was quite convincing. I can't help but think how much of that was thanks to having speakers in such a large well treated space in combination with speakers with unusually wide dispersion. 

It might be really interesting to do a 4 channel recording, but with the microphones in 4 directions around the piano and using speakers back-to-back to attempt to reproduce the sound. Would this make us feel closer to listening to a live speaker? :) Wish I was in college, it would make a good research paper. 

Best,

E

My bane is the human voice going from soft to loud (dynamics) or vice versa. There has always been a "shadow" made of distortion that parallels the transition. A corruption of the air coming out of the singers mouth? I can't explain it but I always hear it.

Piano, oud, upright bass.

I don't think any one instrument is that difficult compared to complex orchestral pieces.  That's why you will rarely, if ever, hear orchestral music played at shows.   It's always something simple like Bach cello suites, Musica Nuda, some "girl and guitar" stuff.  Well, not always, but most of the time.