Schoeps CMTS 501u by scarolinus
I've used a CMTS 501u for a variety of orchestral, chamber, and yes, solo piano recordings, over the years while recording and broadcasting NPR programs. It never let me down. It is an awesome microphone, well balanced and clean, at least in natural acoustic settings. I never had a chance to try one in a studio session so I can't speak to that. Based on location with various instruments, including harpsichord and a Stradivarius, I would expect it to be superb on strings, guitar, even some brass.
I guess "sweet" is as good a description as possible.
Now that being said, like any stereo mic, or pair of mics used as a stereo pair, it can be touchy about placement. If you can get your hands on one in good condition, take the time to listen and work with it, and you will have a winner.
So far as I am concerned, it is still the standard by which all newer design stereo microphones should aspire to match.