Interesting... so it was the crappy speakers that were used causing the problem...

That's a handy tip to know for future!

Mind you, having good speakers in pinballs does make a big difference to the sound.

Well, yes.  But the strange thing was they were brand new crappy'olla speakers.  The new (old) ones were just single cone cardboard $3.00 specials.  The only reason I bought them because they were almost identical to the old ones..But still the humm, just slightly different tone.

With the new ones (dual cone) $30.00 ones from Jaycar - really did make a big difference.  It now sounds the same as what Marks did at the factory. 

So I guess the moral of the story is that if you have a hum, and its not transformer vibration on the cab, and its not a grounding issue, and its not a cap issue, and its not a BR issue then upgrade the speakers in question and you should be better!