I thought for awhlie my Marshall VM 50 watt head was giving me crackles at 12 oclock volume.  But after going to a repair store and trying a different head on my matching VM cab I know it's the cab that is damaged.  I know of at least one instance where the cab fell out of a truck during unloading at a gig.  So... what is most likely the culprit, something needs resoldering?  The cab has so many screws on the back seems they really don't want you opening it.  the problem occurs when the lower notes (E or A) are played open you hear bad crackling.  sometimes it will cut in and out.  

thanks

Not that they don't want you opening it.  They need screws every few inches to make it very tight and prevent strange vibrations from occurring.  Use an electric driver or a variable speed reversible drill with a phillips head and you can get them all off in two minutes.  Then you can inspect for damage or a shaky connection.  You may need to replace a speaker or have it repaired obviously.  But you might find a spade connector loose which you fix in a couple of minutes.  Sometimes you need to crimp one a bit tighter cause they can be loose sometimes.  But you need to get in there to have a look.

yeah def open it and have a look inside. the cab could be cracked or anything and you wouldnt see it from the outside.