I have a heavy solid core door under which my acoustician placed an OEM product which drops out of the bottom of the door when the inside of the door touches the adjacent door frame. Makes the room almost totally soundproof
Caeser - these are some of the best seals - go for the highest rating for jamb, head and threshold - it is best to do a threshold to get a positive seal as the drop down models do go out of adjustment http://www.zerointernational.com/
normal solid core doors are generally not dense enough ( I imagine the units Steve has used are special acoustic doors) - you need at least 50Kg per sqm - something like two layers of 25mm MDF with green glue between would be good - you can get a complete tested system of frame and door but the cost does go up and the quality of installation is the deciding factor in performance
As Phil said, I have 2" heavy solid core doors in the studio with Zero International seals all around and Cam-lift hinges. I even installed "communicating doors" with 8" air gap which goes outside. When we first installed them, the neighbors said they could see band members going in but couldn't hear a thing! YES!!!! It works!!