Was the hum there before the tube change? Did the old tubes work- no hum? If one or more of the old tubes was bad, you could have open screen resistor(s) or some other problem. I'd check pin 4 of all the output tube sockets and make sure you have screen voltage at each. If the old tubes worked, you may have gotten a new defective tube or tubes. Sadly, it's not all that uncommon these days.
As far as setting bias, the proper way would be installing 1 ohm cathode resistors for measurement, using a bias probe, or OT shunt method. You would also be able to tell if a tube is bad or they are horribly mismatched. Just setting the bias voltage doesn't tell you much about what's actually going on.