Tube amp is loud at zero volume

it's hard to tell without seeing a schematic . . . but my
first suspicion (knowing nothing about your amp)would be
that the volume pot is broken. I've had it happen before on
a guitar amp - the wiper separates from the shaft and stays
wherever it occurred. Of course it could be other causes -
need a tech to figure it out.

your volume control rotor may stock inside and in need of replacement.