Our studio has an impeccably maintained, all original SM-2, which is pretty much two KM-56s in one body as a coincident pair.
It's incredible for lots of things... piano, acoustic guitar (although I usually like mono acoustic guitar for most things), etc.
I think you'd really like a partner for your KM-54, if you can find one that matches to your satisfaction for stereophony. If not... SM2s are out there, and compared to a pair of KM54 or KM56 are fairly reasonably priced, many times.
