the shocking truth - cheater plugs

I have 4 amps with 3 prong AC plugs, and a processor similarly pronged. Problem is all of these plug into 2 separate AP boxes - the amps into a PW113a, the Processor into a PowerPakV. I have to use a cheater plug on the hybrid amp, Counterpoint sa-8 to avoid a hum, but not on the Aragon ST or BB or Counterpoint Solid 2. Point is, I have one of those brass discs that has a wire running from the disc to the AC wall plate screw. Whenever I touch the brass disc to dissipate static, and I touch the SA-8, I get ZAPPED!! esp. when wet on the fingers (hand washing after eating, etc.). Am I missing something here? should I be doing the cheat plug another way? coz I certainly wouldn't miss being ZAPPED!!