The sound of the G12M with its ceramic magnet and easy cone movement provided plenty of midrange crunch along with nice compression of the low-end when the speakers were pushed.
In other words, these were fantastic rock and blues speakers and sounded very different from the Alnico type speakers used in other Marshalls as well as Voxes.
The heavy-duty speaker option at the time was the G12H, and with its larger and heavier magnet and stiffer cone had a handling rating of 30 watts (conservatively rated I’d say).
The G12H not only handled more power, but it was also a much more efficient speaker.
Using the same cabinet equipped with either a G12M or a G12H, one will instantly recognize the G12H as being quite a bit louder of a speaker.
Chances are if you ask a player who uses vintage Marshall amplifiers to name their favorite speaker, they’ll either call out the G12M or G12H “Pre-Rola Greenback” speaker.
These two speakers literally defined the sound of rock since the 1960’s and have continued to do so ever since.