@mwl168, I found exactly the same thing. Once I elevated the heaters, the hum was eliminated completely. 12AT7s are at 60V (voltage divider off B+ rail) , and 6SN7s are at -340V (tied directly to B- rail). In all, I did 4 things to make mine totally silent:
Used voltage divider to elevation the filament voltages. +60V for 12AT7s and -315V (or in my case, I just tied it to B- so -340V) for the 6SN7s
I switched to different set of tubes. The ones I started with just hummed. After switching, much less hum
More careful grounding of components
Finally, to get rid of slight hum when volume control was at 11:00 to 5:00 position, I switched from a 50K pot to a 10K pot, which totally eliminated any hum when the volume control was in the mid/high region.