Using Reverb switching to control reverb amount ?

[petemoore]

  Vox AC15 RI amp has a reverb/tremolo footswitch.
  The reverb switch, when plugged into the amp controls reverb on/off.
  I want it to control between two preset [well, one preset setting and one existing reverb knob] amounts instead of on/off.
  The schematic of the amp is a newer ~useless VOX-Korg 'illegible' type, though...designed to be confusing and unreadable...I don't even know if it uses Jfets or optocoupler to do the switching.
  Anyway, for someone with a legible amp schematic...prehaps there's an easy way to switch a resistance in with the reverb pot to make a lesser/more-er switch out of the on/off switch.