Do you know what was changed?

There were 3 revisions to the M1, maybe the input sensitivity was changed on the later models to keep up with more modern kit?
Many power amps that had revisions saw this, some of the early amps were 200ma, then 0.5a and then 1.0a, this makes a huge difference to overall gain for a given input.

No Darling, I've had it months!