Thanks for the great ideas and suggestions everyone.
In this design, I have the filaments referenced to ground on one end of the 12.6VAC, so that it can be rectified to some 16VDC which is what I use for the grid bias. I guess I could have used the plate or screen supply for the fixed bias to have the possibility of adding that trimpot in the filaments supply.
We've already built this amp, only that it was the original version with the tubes in the same side as the components, and there was no hum according to my friend, the guy who put it together and tested it.
Also, I have the largest power resistor mounted vertically. I think I'll do the same thing for the other power resistors, I mean, why not.