Hrm.........The Mk II's have the same engine as the rest of the
B10M's. The Mk II's are meant to have a TDH102KF engine, while the
Mk III and Mk IV are supposed to have the DH10A engine....however,
SBS wanted the same engine, possibly for ease of maintenance, which
well, is lacking anyway, judging by the sounds that these buses
make.
So the only way they would sound different is because of the
different gearbox the Mk II's have (Volvo gearbox, not sure of the
model).