AJ,
The Korg and the Yamaha both have mic volumes from the front panel. The knob on the rear is a gain switch and should not be used for small changes during a performance. Set it and forget it.
On the PA80, you can make incremental changes in the edit menu, otherwise you have to just hold the button down and wait to scroll through the value....too time consuming, and not presise.
The PSR is right on the front panel with a dedicated button - couldn't be easier.