The Sylvania 6SL7WGT I have from 1955 hums less and does sound amazing, the sound stage and instrument separation are amazing compared to the 6AV6 and 6HM5 tubes i've been using. The new production TS 6SL7GT from '05 I have hums a bit more and sounds OK, but nothing mind blowing.
Its really troubling me what is causing the humming, as you tried grounding isn't the issue.
From the interwebs... "6v octal preamp tubes (6SC/L/N7)are more prone to hum than 12v noval preamp tubes (12AX/T/Y/U7)"
The octals are more microphonic than other tubes as well for some reason.
Here is a little test I read about, wrap a piece tinfoil around the whole tube and base from top to bottom and then ground the tinfoil to the chassis to see if the hum goes away.
Also it is highly recommended to have a DC heater when using octals as preamps to try and get rid of the hum.