do you have another cab you can plug you head into? If it were me, I'd isolate the problem before I bought a speaker that may or may not solve the problem.
remove any effects or tuners between the bass and the amp.
Try a different cable.
Try a different bass.
Try a different head.
Try a different cab.
Try a different circuit in your house.
Is there any florescent lighting in the room?
Are there any electric motors or other electric devices nearby?
There are so many things that can cause buzzing. It can be mind bending to chase it down.
I wasn't an MU, I was ATW, made E4 twice.