I just nailed down the source of hum in my Marshall 2203 I built
out of a 2210. I originally used a combination of buss and star
grounding but left the speaker jack connected to the chasis as
well as a few other minior details. I rewired it with everthing
properly grounded and still there was a loud hum. I figured it
was heater induced from that point. I tried moving the heater
center tap to various places on the buss. No luck. I tried raising
the CT to +50V. Still no luck. Then I decided to move around
the twisted pair, foil shielded heater wires at the first preamp
tube. I found a sweet spot that ELIMINATED the hum. It
seems that 1/4 of and inch in movement reduced the hum from
unbearable to gone. In fact, now I can only hear hiss, not hum.

Just goes to show, you can do things 100% correct and still
have to play around with lead dress just to get things working
correctly. Hope this helps.