The "quietness" MAY be due to the fact you are running a different prop. APC props are quieter than MAS props (also more efficient ).

Again, the lower rpms may be due to the prop AND/OR the fact that you are actually using less rpms. You have a 40 size engine (granted it only has the power of a 30 - 35 ) in a 25 size plane. Due to its size, the plane may APPEAR to be going faster than it really is so you are using less throttle to achieve what you perceive as the proper flying speed (a speed you are comfortable with ).