"Trevor Wilson" wrote

**They only knew that there might be a change in the system when the
light changed from green to red.

Which "light" controlled how?

**The light in the listening room.


Why a light? It's existence reduces the validity of your results. The
listeners should not only not know which amp is 'A' and which 'B'. They
should not know whether, at any given time, they are listening to 'A' or
'B'. Listeners will often give a preference when asked to compare 'A' to 'A'
or 'B' to 'B' because they think there *ought* to be a difference! Ideally
they should not even know that the test is a comparative test of amps
(rather than say sources or cables).

David.