Killed my 50/50?

[backyardburial]

I have a 50/50 which has been through some unfortunate scenarios as of late.
At a gig the other week I was running in stereo with my mesa recto cab on one side and a Marshall 1960 on the other, half way through the set my amp developed a huge hum which was fixed when I unplugged the marshall cab and went stereo just into the mesa. I later discovered the Marshall had been rewired so when I thought I was plugging to the 4 ohm output I wasn't. Turns out the owner didn't even know this.
Anyway, the amp was fine for a while but during jam the other night the power in our room was a bit dodgy due to a storm. I turned the amp off...
Last night,turn amp on, it sounds fine untill I get the volume past about 3 on both channels, then the hum came back with a vengeance...

Right, so there are the possible causes I hope? The one strange thing I noticed was that the hum was incredibly loud, (I did get the amp up pretty loud) but when I turned both volumes down, it was still there for a good 15 seconds before sudden silence. The preamp was not even on at this time by the way.
I have a gig tonight, can anyone tell me if it is worth grabbing new tubes, or is this likely as more serious problem?

 

[JMMP]

If it isn't too much trouble, you could take the lid off and see if any components are visibly burnt. BUT DO NOT TOUCH ANYTHING. Lethal voltages are present, and if there are problems in the amp, the voltages could be hiding anywhere.

New tubes may help, and if you are planning o having tube amps for awhile, they won't go to waste. Especially change if any of the tubes look black/burnt where they shouldn't be.

 

[backyardburial]

Thanks mate Ill do that, just wondered really as to how sound can be present when all volumes are down...