Interestingly the SUT in the Lamm is a real weak point. The external, even a relatively inexpensive one like the AnaMighty one I have, is a huge improvement over the Jensen’s inside. I just A-B’d it again this morning and it sound great with the internal but the external is sublime.
The system change that has caused this problem to become noticeable is a switch to SET and higher sensitivity speakers. The amp change came after I first noticed the hum so it’s not a source of the problem.