Damn it, JimL. Must you always be right? First, you intuitively guess a bad DN2540 on the input tail CCS and were right. Then you intuitively guess 12AT7 issue and, again, were right. I put in all of the known good tubes from my prior amp, and the new amp is now acting and adjusting perfectly. Rock solid and steady, even with SE config (cold input tied to ground). And even with the filaments still elevated.
I did not think at first to try that because I had already put all those good tubes in the new amp and it did not help. But that was when I still had a bad DN2540.
JimL, you absolutely rock the STAX world!!
mwl168, jose and mypasswordis, thank you too for your help. I very much appreciate this wonderful community.