So I've got a pro jr that I love dearly, it's really the best all around amp I've ever had once I figured out how to run the rest of my rig with it. When it's cranked up into the sweet spot, though, it gets a faint metallic rattle/ringing sound, particularly in the neighborhood of Ab-D. I suspect a microphonic tube, but I've swapped them all out multiple times to no avail. Any help appreciated. Also considering upgrading the speaker to a g10 gold and the OT to something. Has anyone here had experience with either of those upgrades with this amp?