I had the same concerns about altitude thirty years ago.  I have had
no problems operating a Reproduco piano/pipe organ, an Ampico upright,
a pumper, or a Steinway Duo-Art grand here at Lake Tahoe at 7000 feet
MSL [feet above mean sea level].  Robbie's comment about psi fits --
differential pressure seems to be the key here.

Ray Bauer
Tahoe City, Calif.