The bias+balance pot setup is easier and slightly less intrusive for a
2-tube output.
 
Find the value if the bias injection resistors (in most Fenders,
they're 220k)
 
Lift the common end of both resistors.
 
Parallel each resistor with one of simlar value and wattage.
 
Wire a 200k to 250k linear pot (something near the value of the
bias injection resistors) so that the wiper goes to the spot you lifted
the resistors from, and each end goes to one of the resistors.
 
With the balance pot centered, you're very close to a stock setup.
Start your bias adjustment there, then unbalance as required.
 
HTH!
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This setup works really well! but I found I had to add a 1M resistor
from each output tube grid to ground to get a noticeable balancing
action. With a 12AX7 driver tube and the 1M resistors, you get
sort of a combined AC-and-DC-balance function that I was real
happy with. I just replaced the 220K resistors with a 1M linear
pot, with a 470K resistor from each end to the wiper and the wiper
connected to the bias feed point.
 
Ray