Dave said, ".......Sikorsky stop work on the craft after it lost the competition."
There was no competition, Dave. The XH-59 fulfilled its contract, a follow-on was discussed but not funded, and later (much later!) the X2 was built. A sister contract at the same time was the XV-15, both were administered by the same Army engineering group.
Ian, I believe what I once posted was concerns that the small prop on the X2 was in some pretty poor air behind the fuselage, so that it wouldn't be as efficient as one would hope.
The RAH-66 Comanche was pretty smooth, vibration was never an issue in any maneuver or speed I had flown, and there was no vibration absorber or suppression system ever installed. This is typical of stiff, thin gunships, where the structure is very rigid. Big open cabins with door cutouts and flat spans breed vibration problems.
Pilot lore that "bad" rotor heads produce vibration is just that, mythical. All rotors produce vibrations, what we feel is how the fuselage reacts to that vibration.