I’ve had both. The M-60 had a different sound, I’d say a little more bright than the M-4. Both were very nice though. Good bass control either way.
I paired them both with an unrestored C-2a and they both seemed to get along well. I don’t have either anymore so won’t be able to see what they’re like on a restored example.
I’d say it would have more to do with your speakers and how the amp worked with them.
Personally I like the M-4 quite a bit better than the M-60. Not that the M-60 sounds worse it is just that I love the build quality of the M-4. It is truly a robust and solid built amplifier that has a pleasant signature to it. The M-60 and M-80 amps I’ve been into always seemed like they were a bit more fragile in construction and the components often saw some duress for whatever reason. The M-4s I’ve been into always seem like they take a licking and keep on ticking.