Little Mark Tube 800 Head Buzzing/Grounding Issue

[bassTribe]

I bought this Mark head just over a year ago. I gig with it a lot and love it. I have only had 3 problems with it causing some buzzing (maybe a grounding issue?). Tonight we couldn’t figure it out so I had to run through a DI. Has anyone experienced similar problems? I checked the ground switch on the back, tried different xlr’s and other cables. I don’t understand the electronics well enough to problem solve. Suggestions, similar experiences, and solutions all welcome. Thank you!