How did you turn Hyper-V off? In order to use other hypervisors like Virtualbox, you either have to have it uninstalled or you have to disable it using a custom boot routine. The easiest way to do the latter is to use BCDEdit to duplicate the existing Windows 10 boot entry and then set the "hypervisorlaunchtype" setting in the duplicated entry to "Off" -- then you have to boot the system using that option instead. Just making sure all Hyper-V VMs are off and even stopping the various Hyper-V services on the system is not enough to allow another hypervisor to start.
