Hi! Welcome to the place. From your post, I'm assuming you have sufficient experience to know how to work on high-voltage devices. Regardless, BE CAREFUL!
The 100R/5W in series with the 20R/1W will split the power dissipation proportionally to their resistance, in other words, they will each dissipate max power at the same current. So they'd be a good match-up. But 120R in a circuit where 180R sounded good may not change the sound enough to please you. The reason I'm saying that is I've been playing around with a 2xEL84 amp of my own, and have gone from a cathode resistor of 120R up to 300R and back (I think 150R is in there now) to get 'that sound'.
If 2W resistors lasted *any* amount of time in that amp, you could string all three 100R+20R+50R together and the 50R would be OK. You could experiment with different combinations, and when the magic cathode resistor value is found, order and install a 5W wirewound resistor of the desired value for longevity. I'd advise keeping an eye (or IR thermometer) on the 50R resistor if you do use it, while testing your changes.