Sound muffled haldway though gig

[pops]

Did a gig last Saturday. One where you can turn it up. 1st half everything great. Put it on standby for 15 minute break. As soon as I switched on for the second set it lacked power and top end on both channels so I tried going in the front end direct without the pedal chain but it was the same. I struggled though the second set without any change. I've just rigged it up at home and really pushed it and the sound and dynamics are back to normal. I was hoping for something obvious like swapping tubes if the problem had still been there, but it isn't. Any ideas? I have a gig this Saturday and don't want a repeat of last Saturday. Help much appreciated on this...