Does the rating suggest that there will always be some hum and that this is normal? Does anyone know how this rating works at a practical level? Cary didn't have much of an explanation except to say that yes, some hum is normal.
Your speakers appear to be rated at 89db/1W/1m. Neglecting room effects, and assuming you are using triode mode, it can be calculated that 84 db below 50 watts into the two speakers would result in an spl at a 10 foot listening distance of 15 db. Since the 84 db spec represents hum and noise combined, and since the spec hopefully provides at least a little bit of margin, the 15 db figure should be at least several db less, with respect to hum. I would expect that spl's of around 10 db or so would be completely inaudible in a typical room environment, particularly at deep bass frequencies where our hearing mechanisms lose sensitivity at low volumes.
So my suspicion is that the amp is not up to snuff, even though Cary checked it out, although your statement about the improvement that resulted from changing the preamp's power source suggests that external factors are contributing to the problem as well. Cary's statement that "some hum is normal" is simplistic at best, because it doesn't take speaker sensitivity into account.
In saying this, I'm assuming that the hum is at 60 Hz and/or 120 Hz, as opposed to a higher frequency buzz, and that there are no nearby sources of low frequency emi that might be coupling into the amp through the air. To reach a firm conclusion, also, you should verify that the hum still exists when shorting plugs are placed on the amp's inputs.
Another thought is that if you are presently using the amp's 8 ohm taps you could reduce the hum level a little (close to 3 db) by going to the 4 ohm taps. I couldn't readily find an impedance curve for the S8e, but I found some indications that its 8 ohm nominal impedance probably drops down to 4 or 5 ohms at a lot of frequencies. That MIGHT mean that sonics on the 4 ohm taps would be at least as good, or possibly even better, than on the 8 ohm taps.
Excellent comments by Jim, as usual. I would add, though, that I don't think dedicated lines will help a 60 Hz or 120 Hz hum problem very much. They would be helpful mainly with respect to higher frequency hash.