Try a choke instead of the resistor.
You already discovered it, but keep in mind that there are MANY sources of hum in an amp, and each one is different. You have to solve each source independently. In other words all the filter caps in the world will do nothing to fix grounding problems, and elevating heaters or putting them on DC will do absolutely zero for power supply ripple hum. So hum is not generic.
SIngle ended amps naturally hum, for the reason you discovered. They usually take the B+ for the power tube from the first filter. So you add a filter stage. Fender has a service bulletin from years back, telling service centers not to try to"fix" the hum in Champ amps, it was normal. It would not be covered under warranty.
Look at Fender Champ drawings. The 5C1 has the power tube drawing from the second cap, like your solution. (is that where you got 500 ohms as a value?) The 5E1 uses a choke there instead, but still draws off that second node. Then the 5F1 does what everyone else does, takes the power tube supply off the first cap. Now the amp hums, but they save the cost of the extra cap and the choke or resistor.
The best approach would have been to find the source of the hum and just solve that, rather than guessing at several solutions for possible causes.