Here is a thread I found on a typical fix:
https://forum.speakerplans.com/ev-p3000-protection-issue_topic33159.html
An amp this powerful I do not recommend that it's your first repair. I haven't seen the schematic, but it looks like it's available for a donation and from the generous person at ETO here in this thread.
I worked on a commercial amp where the components "puddled" on the circuit boards. No shop would take it in and there was a total of five asked.
It's likely that transistors that are in parallel have to be matched in current gain.
I have done extensive repairs on high power amps and other high power things such as 30 KW (15,000 V @ 1A) power supplies and 100 KV @ 0.1 A power supplies and repair of tube devices such as 1 KW 15.56 MHz RF transmitter. My tube experience stared when I was a 10 year old kid or so. My amp experience started when I decided to build one (google The Leach Amp) in the mid 80's from scratch. I started doing amp repairs for a local consumer electronics repair outfit. I typically only did their hard ones. One particular amp had come in for a warranty type repair and I had never worked on it. I found a minor design problem and all has been well.
Not replacing all of the defective components or not matching the transistors can result in premature failure.
Minimum tools are oscilloscope, signal generator, ability to measure transistor gain, and a Variac and a DVM and the service manual obviously helps.
Nearly all the catastrophic repairs require output replacements, driver replacements and attention to the bias network. The small resistors <300 ohms always have to be checked.
The xistors are about $2.50 to $7.00 USD, but you have to buy enough to assure a decent match in Hfe.
There is 12/side or the possibility that 3 have to be matched. That's also judged by the 6 emitter resistors.
The rust suggests moisture and that's in line with the cause presented in the link I posted. It's even possible that the transistor mounting screws rusted and shorted.