Unitiqute speaker protection fault

I have a Unitiqute driving a small pair of spendors in a office. It's hardly ever used but I have noticed that one channel drops in and out at around the 40-45 level and sometimes doesn't even work at all. I have checked all connections and even changed the speaker cables and swapped the speakers around but it's still doing it. The thing that's really confusing me is that if I switch the loudness on I can take the volume all the way up without it dropping a channel. I can only imagine it's to do with the protection circuit but with the loudness switched on the amp will work harder so you would think it would not even make 30 before it drops out. 

I was wondering if anyone had any ideas or able to point me in the direction of the offending component at fault is it's not worth sending for repair.

Thanks in advance