Possibly the power supply is a bit borderline and extending the cable results in a slight voltage drop on one of the supply lines.

Having said that though, I would have thought the empeg guys would have designed in enough overhead that 7 extra inches of display cable wouldn't have made much difference. Turning on the LEDs will be adding some extra load. Again I would think there'd be enough overhead designed in.

Another possibility is external RF is somehow affecting things but your description implies power supply issues. Alternately, the power supply issues together with the extended cable cause it to be more sensitive to RF.

I'd be looking at the supply lines on a CRO/DSO myself but you probably don't have that sort of equipment available to you.