Wow, Chefelle, talk about different experiences.
I've never had my Pro 600 even labor under the load of a double recipe for whole grain breads, let alone have the gears grind and a smell of burning motor.
In fact, I'm surprised you're experincing that, because the Pro 600 supposedly has an automatic shut-off system if it starts to overheat. I've never needed it, so don't know first hand if that feature really works.
I originally got the Pro 600 because I have an antipathy to tilt-head mixers and wanted the bowl-lift type. But the more I use it, the more I'm impressed with it for all sorts of other reasons.
But, as I've said before, unless a lot of bread making is in the offing, the Artisan is all the mixer most home cooks will ever need, and I'd no more recommend the Pro 600 for that purpose than I'd suggest that home-cooks need a continuous feed Robot Coupe as their basic food processor.