Here in Manitoba the rut usually peaks around the last week of Sept. or 1st week in October.
The weather seems to play a big role in moose coming to calls. A few nights of frost and some clear calm weather can make for really exciting action.
I generally use a mix of bull and cow calls. In my experience, using only bull grunts especially if you sound like a dominant bull, can make the smaller bulls a little skittish. Since we just want moose meat and will shoot any legal bull we usually start with a few cow calls and then throw in a few bull grunts a little while later. The grunts often work well if you get a bull interested in the cow calls but just doesn' t want to commit. Throw in a few bull grunts and that will often do it. Especially if it is a more dominant bull.

Hot windy weather and you can pretty much count slow action.

Calling moose is really exciting and when you get one in the right mood, look out!

To answer your question the more mature bulls that I have heard and seen have had deeper and more throaty sounding grunts than the little guys.