Ah I get ya now, external or separate Psu connector

Not really.  All the i/o & power are done on the same connectors as are on the back of a 1073 etc

Remember these compressors were not really designed as stand alone units.  They were designed to be plugged into slots on a desk, & as such have the same female 18pin edge connector that all the modules of this era have.

I have just replaced the four rectifier diodes D7 to D10, they seemed working normally, but I replaced them anyway. Still nothing, heater voltage drops to 0.3V under load. Does that mean that the transformer is the culprit?

Checked headroom and THD.
Max input at onset of clipping is -11dBu. It should withstand the kick drum mic (hopefully).
THD is not terrific with about 0.1 to 0.2% for output level between +4 and +15dBu. Goes down with level but hard to measure cause noise interferes.
After all it's not so bad for a circuit with so little NFB. Doing what I can to comfort myself... 

Yes, you can generally hear the difference of polarity. 

Generally?
No you can't. Generally you can't hear any difference at all when you are soloing 1 track and flip it's Polarity.

Lennie's problem is not related at all to the people that discuss they can hear absolute polarity, he complained of low end loss.
The problem is somewhere in the circuit don't chase ghosts

Hi Khron, hi everyone!

I've just replaced EVERY component after the rectifier diodes and the problem persists (as soon as the load is connected the voltage goes down to 0.3V. (By the way, voltage after rectification is about 12VDC)

No there is only the transformer or the rectifying diodes D7-D10 left. What would you recommend trying next? I would really really appreciate your help.