I'm wondering if the tires have 'flat spots' on them from sitting/storage. If so, they should smooth out after a bit. What tire pressures are you using? 36F/40R is good for the Speeedmaster with OEM Metzlers, not sure what tires are on the America.
Do you feel the vibrations at all speeds or only at highway speeds over 80 or more km/h? If it is flat spots, they would be felt over a greater range of speeds.
Not quite sure what else to suggest.
H.
