Ashdown superfly output issue

[mattshortridge]

Hi all, I bought an Ashdown superfly on ebay that seems very clean and generally works/sounds great. I built 2 cabs to go with it.
The problem is that each output works fine individually, but connecting both cabs--which you need to get the full power out of the amp--causes and occasional loud mid-range whine that is not quieted by the volume control. The whine can be made to stop by jiggling one of the speaker outputs. They are Speak-on/1/4 inch combo jacks. Something to do with connecting both speakers.
I'd love some advice or suggestion.
Thanks!