Separating the wheat from the chaff in an advice column can be helpful. No offensive intended here. Working mostly with DC circuits and some AC circuits, I found out a lot of lies that sounded right but would lead you in the wrong direction. >>>>"Ground is ground the world around"<<<< was one of my favorites. Sounds catchy and true, but no, it isn’t.
I am sorry that the DC blocker didn’t work for you. Would have made things simpler. One thing that you mentioned was that there is Still Hum when using the amp on a different circuit in the house. That definitely points to the amp at this moment. Replacing the tubes may be a good idea, yet I would be inclined to start only with the rectifier tube if the amp uses one.
Try to isolate One Thing at a Time. That is what we did in my job, and though it might seem tedious, it won out in the end... most of the time. As others have said, it can be difficult to find a hum problem, but at least there are a few things that you can do on your own.
I look forward to following your progress.