Regarding the Tempo problem from Gary Heckman, it sounds to me like
the slide valve for stack cut-off during rewind may be set too far
forward, so that it 'kicks' in during faster tempo.  It should be a
relatively simple matter to re-adjust the valve so that it works on
rewind only and allows proper tempo adjustment during play.

Perhaps it's as simple as re-adjusting a screw-type lever rod collar,
or a pallet valve is being prematurely activated by an incorrect
adjustment depending upon the stack cutout system used by the Knabe,
if it is pneumatic.

Just a couple of thoughts, as I have come across similar faults and
they are very easy to rectify.

Kind regards,
Paul Brennan