Plaster Walls

Plaster/lath is much denser, more rigid and thus: less resonant, than the Sheetrock over stud construction, of later homes. The former retains less energy/returns less to the room, than the latter, which I've personally found; makes those rooms easier to treat. The shape and dimensions of any given room will have more bearing on the Sabine times, absorption or diffraction treatments necessary, for good audio reproduction. Plaster/lath also better reduces the sound & noise that is transmitted to/from adjacent rooms, or the great outdoors.

Yeah plaster should be fine. Plain sheetrock is the worst. I had to move back to the basement with stone walls because the modern sheetrock was thrumming. It you do ever decide to sheetrock, put it over the plaster.

Plaster is preferred for its acoustical properties. You did well. Rodman is right. Drywall is a step backward.

Rodman99999 nailed it (pun intended). "Plaster/lath is much denser, more rigid and thus: less resonant"

I agree with all of the above. I have had systems in both plaster and drywall, and I think plaster is preferable.