Without differential scope amplifier/probe, audio out is missing something
in the bass range about 70-80 Hz. Line-out signal faithfully follows the
bass guitar input signal . Shure microphone, capable of those frequencies,
monitoring the sound from the speaker, does not match the input at those
sort of frequencies. But is it a failing of suspension or something of the
speaker or some inductive/capacitive interaction with the amplifier when
loaded with the speaker?
Not yet monitored the main rails to see if dipping on this bass amp.
I will then be subbing the speaker, as a test, but it still would not
determine whether an amp or a speaker problem , speaker is original to the
combo. Any simple component to put in series or parallel with the speaker to
check if an output matching problem ?