  A microelectromechanical oscillator with a 0.73 $\mu$m gap structure is
employed to probe the surface Andreev bound states in superfluid $^3$He-B. The
surface specularity of the oscillator is increased by preplating it with 1.6
monolayers of $^4$He. In the linear regime, the temperature dependence of the
damping coefficient is measured at various pressures, and the normalized energy
gap is extracted. The damping coefficient increases after preplating at lower
pressures, which is attributed to the decreased energy minigap of the surface
bound states. The device is also driven into the nonlinear regime, where the
temperature independent critical velocity at each pressure is measured. The
critical velocity is observed to increase after preplating at all pressures,
which might be related to the increased average energy gap. The observed
behavior warrants a microscopic theory beyond a single parameter
characterization of the surface.
