@dhcod I have real trouble believing that the Jensens are actually worse than your outboard SUT. But in order for them to work right, the Jensens must be loaded correctly at their output. I expect Lamm has a provision for that. The loading changes from cartridge to cartridge and Jensen has published a pdf on their website with the correct loading values based on your cartridge.
It may well be a power transformer associated with the SUT that is causing the hum. IOW look at associated equipment to see if there is a power transformer nearby that might be causing this.