How about the power socket of the amp? Is it two-pin type, or 3-pin type?
If 2-pin type socket, then you're using correct power cord. It means your device is double insulated type meant to work with floating ground. I remember the Marantz CD6003 had a power socket which was physically similar to IEC 3-pin male socket but ground pin was missing. But this quite normal - just check any DVD or Blu-ray player... They're all powered by 2-pin power cords.
If 3-pin type socket, then you're using wrong power cord. AFAIK, a 40-50W amplifier must be run with three lead supply, so my guess is you're using the wrong power cord and operating your amp with floating ground. It's a serious safety hazard, BTW.
On a related note, I see people randomly use European/Schuko power cords to power devices at home, just because Schuko male connector plugs into Indian wall sockets. European power cords must be used only with matching wall socket which provides ground contact. Otherwise it is a big safety hazard.
And yes, the funny thing about protection issues, they always seem to disappear at the dealer's place or at the service center, they also tend to disappear momentarily when the amp is back from repairs, only to reappear again if the root cause is not identified and rectified.
