Well after a long night of listening, I think we have a winner. The 518ia is a great amp, by any measure. Explosive dynamics, fast, tonally rich, and plenty of power to drive the Super V.
Now the bad news, hum. For the first three hours the amp was powered up there was ZERO noise. It was so quiet, you didn't even think the thing was on. Then a steady state hum started. I don't think it's a ground loop because it would have been there from the start. It is present on all inputs and even when the amp is muted. I don't think it's a bad tube because the volume is equal in both channels and doesn't change with attenuation. You can't hear it when music is playing, but you can hear it from anywhere in the room when there is no music.
I'm usually pretty good at diagnosing these things, but this one has me stumped. It's not a deal breaker for me, the amp sounds too good. But I'd certainly like to figure it out.
Bottom line, as good as the CJ is, it will be going on the auction block.