re: Who were some of the bands that smoothed the transition from hair metal to grunge?

Sub Pop - the label.

Malfunkson, Green River, Mudhoney all took the Seattle Underground Punk scene and with Mother Love Bone, Soundgarden, Nirvana, AIC . etc...It became Grunge. I was reading an article that spoke about how the bands hated being called Grunge music but they said what are you going to do.

But Sub Pop brought many of those band the publicity they needed.