**UPDATE**UPDATE**

So I thought the hum was gone. Not quite. This AM I turned on amp after being off 6 hours or so. Slight hum, no matter what else was disconnected, so long as unbalanced between 990-7700.

SO.......I got out some old mic cables (XLR(F)-XLR(M). And as I posted elsewhere: **VOILA** no hum !!!!

There is one variable and that is that the amp had been on for an hour or so. I'll leave it off awhile and if the balanced connectors do the trick that's it.

However there is one wrinkle: Been using an EQ between 990 and previous (755). It's an Alesis DEQ830, which you can use balanced or unbalanced.
So if I want to use it I'll make XLR-TRS adaptors for it.....or....if outlaws tell me it's OK to try (and I'm waiting for 9AM to talk to Scott) I'll prefer (for ease and cost) to go unbalanced if they tell me I can do that and have a XLR-XLR balanced in to carry whatever reduces hum. This will be tried later this AM.

Tune in again and I'll write the results.
One thing to remember if you have hum: it's not the amp. These are absolutely dead dead quiet.