>I still haven't ruled out observer/user error, since of five similar 
>setups,
>only one is really having this problem.  However, a bad amp could
>cause that as well.
>

Can you exchange the amplifier from a "good" setup for the "bad" setup?  
Likewise the sound level meter?  If the meter is battery powered, exchange 
just the batteries between a meter from a "good" setup and the "bad" setup.

What you are describing does sound (no pun intended) like human error is 
getting into the measurement.

Rob Young


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