Issue with predator

[Romegoth]

I had a hum issue with my predator, on closer inspection I found resistors R12,R6,R105 and R112 (there may be others) to be cracked and signs of heat. The amp is a little old now (version 3 2005) and still has the original valves. Any ideas as to where I should start troubleshooting ?
pictures attached
regards
Mike

 

[gavinbirss]

 

[gavinbirss]

 

[Schalk]

 

[Schalk]

 

[Romegoth]

 

[Romegoth]