An Ashdown MAG 300 amplifier with a reported loud intermittent hum fault on the speaker regardless of volume control. Initially the first point of call is to measure the speaker terminals for any DC appearing on the output. Any significant DC on the speaker output will generally make a hum. This amp all seemed clear. A great little workshop tool is a small insulated screwdriver, lightly tapping on the circuit board often reveals all kinds of issues, microphonic capacitors, bad joints, broken legs.
Suspecting a storage capacitor issue because of a) the lack of DC on the output, b) significant hum the power board was unscrewed.