Lower power on left channel

Hi all, 

Does anyone have any ideas or come across this before?

I have a Primaluna Evo 300 pre-amp with a Primaluna Evo 400 power amp, using G/L KT 88 tubes. The other evening I was sat there playing some music and suddenly thought that it did not sound right, as I stood up and walked towards the centre of my speakers I noticed the the left speaker was down on volume compared to the right speaker. As I went to turn the system off I noticed that the KT88 tubes where down on heat to what they normally are.

I can not do much now till after Christmas but any help would be much appreciated

Merry Christmas 

Steve.