Peavey Head - Limiter Clipping at very low Pre-Gain?

[bpbpbp]

Hi,

I've been using a Peavey MAX160 head in my rehearsal room for people to use. It was a cheap purchase and I expected it to get some abuse but in only a week it has developed a problem

With hardly any input gain dialled in (no.1 on the dial!) and a passive bass at half volume on the volume pot, it is clipping the limiter (and making a click noise) when the guitar is played anything other than really softly.

A friend said something about people running an active bass through fx pedals can cause this, but I've never come across that before.

Has anybody experienced symptoms like this before?

Thanks!