Channel imbalance in the Crack tends to fall into 3 categories:
1. The source/headphone combination is overly sensitive, so the level control can only be turned up ever so slightly for a very high listening level. Since low-level tracking is a difficulty with these parts, the resistor mod can be used to adjust where the optimal listening position is to reduce the imbalance. It is still possible that your setup is overly sensitive, can you use at least 90 degrees of volume pot rotation?
2. Sometimes the individual triodes inside the 12AU7 and 6080 can be out of balance. It's rarely the 6080 that causes this kind of problem, and borrowing or trying a different set of tubes can let you know if this is your issue.
3. You have a cold solder joint that is obstructing the signal somewhere in the amplifier.
One step you can also take is to play a 60 Hz tone into both channels of your amplifier, then measure the AC voltage between ground and these points with the level control half way up, then repeat with the level control all the way up:
1. At the center pin of each RCA jack.
2. At the middle lug on each level of the volume pot.
3. At terminals 6 and 10 in the amplifier.
This will give a decent breakdown of where the imbalance is originating.
-PB