Just for a test, before I do anything else as previously suggested I ran a new molex from the speaker out on sound board with new wires straight out of backbox a couple of metres to speaker sitting well away from machine.  No different.

So I guess this eliminates noise introduced  from loom and speakers being next to DMD.....

I just found the bulletin you were refering to Nino too.

http://www.sternpinball.com/downloads/sb133.pdf

I have no idea whether this would work on a D.E Soundboard for LAH.  I would defer to the experts on that one.