I went back to Jaycar this morning and bought some slightly less crappy 4" 4ohm speakers for the display panel.  Although the hole patterns are the same, the new ones have a tweeter which sits proud of the mounting.  I've made some wooden spacers to slip in, but I will try to get some circular spacers (mdf) to make it look a bit more professional.

I also installed some ferrite EMI things on speaker wiring.  They didn't make much difference but the speakers did.  The loud humm is now a slight humm and is now equal to that of my Simpsons, which is only slight.  I still get the G.I noise but I'm satisfied that is a factory feature...

So I think the issue is resolved now!

Thanks again for everyones input...