What’s wrong with these McIntosh Monoblocks?

Test pilot is correct. Hooking them up without a load is a no no.

I am an electrical engineer who restores vintage audio equipment as a hobby.  The previous posts are not really correct - the MC40 does not need a load connected to verify the DC circuit voltages, as the circuit is perfectly stable at idle.  Just do not connect a signal at the input.

But the real issue here is that you plugged in ancient amps into the wall outlet!  YIKES!  You are lucky they are well fused or you might have gotten a fire on your hands.  Thought I suspect previous people made the same mistake.  You needed a variac at a minimum.

I can tell you all the caps are shot.  The electrolytics are probably shorted.  Hopefully the output transformers are ok - that's the most worrisome part.  Replacing all the caps is a huge job (I've done it 5 times and don't want to do it again lol).  But it would be worth it.

Thanks for the response, but I have a hard time understanding how to apply a variac to this equation. Says the amps are rated up to 125v, so I just plugged them in. But evidently there’s much more to this than I thought.

If fuses blow right-away, chances that you have bad output stage are dominant and you will need to inspect output stage components such as DC supply literally one by one if you don't have schematics certainly starting from output tubes. After testing one or few bad output tubes, don't be in the rush placing new tubes!!! Inspect components around tubes such as caps, resistors, large filter caps and resistors around large filter caps.  

Take the amps to a tech who can work on them any of a number of things could be wrong. And as said above always use a variac and ramp up the voltage slowly. A sudden inrush of current can do damage to long unused electronics.