Recently I started experiencing signal drops and crackling in the left channel. Usually it goes like this: I switch on the amplifier, after some seconds the right channel sound is ok, the left channel is almost dead with some low level crackling. Then I start to hear some low level music along with the crackling. While the amp is warming up the normal level comes and goes along with the loud crackling. When it is warmed up it is still unstable.

I switched each and every tube, all work perfect in the right channel and crackle in the left one. I even replaced the early Christmas mod caps with a different brand, but it didn't help.

Any ideas?

Already tried that, same thing, same left channel. I write here just in case it is a known problem. Looks like a bad contact somewhere.

This problem has "volume control" written all over it.  I'd be very surprised if it was anything else.  Try rotating the volume control, does the crackling follow the rotation?  Try rotating the volume control back and forth a hundred times or so (probably with amp off).  After this, is it better?  Sometimes with the volume on full the resistance element is virtually bypassed...does it still crackle when the volume is set to full?  

If you find it is the volume control, they sometimes can be successfully cleaned if the phase of the moon is right and you are having a lucky day.  I've had more luck with WD-40 than with any other potentiometer cleaner.

Dan