Treble lower on r/h channel K5881 mk2

[shadders44david]

Hi there, Wonder if anyone could offer some advice on a problem i`ve recently noticed on one channel on a K5881 mk2. The treble and output are lower on the right channel than the left. I have swapped the outputs over to confirm it`s the amp at fault. I have tried swapping the valves over, cleaning the valve pins and sockets, checked all resistor values, checked live voltages, resoldered the right channel tag board all to no avail and all the capacitors are fairly recent. I have now exhausted my knowledge and would welcome any suggestions. Regards, David.