If your front rotors were warped, normally the steering wheel would quiver from side to side when braking. The vibration of the car (vibra-bed) is normally caused by the rear rotors. If your rear rotors and pads are close to new also, you might try the cut out shim kit that Volvo sells to help with the rear brake vibration. Your discription of the vibration and how to cause it doesn't sound too bad. When really bad a shot trip up the freeway with 5 to 6 pedal applications will normally get the car shaking good on demand, and it will continue until the rotors cool down. Try the cut out shims and let us know how it went.