Got a Peavey Mark IV bass head about two months ago and i saw that it worked, took it home and tripped on the power cord. So now I'm making it a unpluggable plug and taking the thing down.

My dad says the transistors are losing their heat seals, anyways, they are Peavey name "SJ6357" aka "MJ15003" silicon transistors. Would Replacing them with germanium transistors be possible and tonally better for say doom, punk, and aggressive classic rock
silicon is better than germanium, germanium has a tendency to change depending on temperature and humidity. tonally they're pretty much the same when the germanium isn't being killed by heat
1978 Peavey T-40 -> Ampeg Micro-VR - > Ampeg SVT210AV + Ampeg SVT-15E
The markIV is a great sounding head, I still miss mine. Unless your Dad is a real expert I'd leave the output transistors alone if it is working, If he is an expert get him to remount them with new heat compound. The only problem I ever had was with the power smoothing caps which i replaces after one burnt out.

There's little point in trying to mod an amp like this. Look to experiment with effects or other speakers/strings etc if you want to get a different tone