Under the Goodmans Axium 15Ω speaker was the driver board. It had a minor issue … it was faulty! Replacing the 40407 driver transistor and all dried up capacitors, affected a cure. Lovely sound and it uses a novel idea at the time, positive feedback on the amplifier;- when the speaker draws more current, as it is not so efficient at odd frequencies, the amplifier detects the extra current and adds a fraction more drive, equalising the linearity of the overall sound.