Hi Ben,  Unfortunately, the Ampico B is too complex to analyze with the
data that you have given.  It this point, only educated guesses can be
made.

My guess is that the entire system is leaking to the extent that valves
2 and 4 are ineffective unless the stack spill valves are closed.  The
stack spill valves will close when valve 6 is activated.  Once that
occurs, sufficient vacuum is available to show step increases with all
valves.

Since the "Sub" valve reduces the stack vacuum, the correct operation
of that devise is meaningless, given my premise is correct.  With
valves 2 + 4 + 6 open on both treble and bass, and the slow crescendo
valve also open, does the stack approach or exceed 40 inches vacuum?
Any value substantially below 40 indicates a weak pump or a leaky
system.

Good luck!

Bob Taylor