Getting electric shock from Rotel RA931 Amp

Hello all,

Well, as above really, I'm getting a very very small electic shock from the metal facia of my amp.  Amp works fine.

Its connected to a Squeezebox and a pair of B&W 601 speakers.  

Any ideas what could cause this?

Detailed Info:

When the amp is ON, if I run my finger along the front metal fascia I can feel a very faint tingling feeling in my finger.  I can't 'feel' it with a stationary finger. It feels exactly like the metal facia has a textured finish on it (but its actually smooth), but when turned OFF it feels very smooth.  Hopefully that will give you an idea of how small the 'shock' is.

I can't detect it when touching the rest of the amp body, just the front fascia.  

I'm turning it off at the mains overnight just as a precaution.  I just don't want to damage the speakers as I love them!

Kit (minus squeezebox) was purchased second hand about 4 years ago and this is the first time I have noticed this.

Any help on what to check would be greatly appreciated.