Uncle Dave,
An expression pedal and volume pedal are the same. I might have misunderstood you when you said that an expression pedal just makes changes to the volume. An expression pedal can be used to get a completely different type of feeling to the sounds. THe expression pedal can be pulled back (volume lowered) just as the chord is being pressed, that gives the effect of the sound being sucked in, the opposite of pressing hard (velocity) on touch sensitive keys.
Star