Here's what I did recently to trouble shoot a problem that I needed to narrow to the culpable component. It'll work for you too.
1. Swap the L&R interconnects from your preamp to your amp. In other words, plug the IC coming from the preamp's L output into the amp's R input, and vice versa for the other channel. Does the channel imbalance now switch from how it was before the swap? If the problem lies with anything upstream of the amp, it will. If it doesn't, then the amp is to blame.
2. Restore the preamp to amp interconnects to normal, and then swap the L&R interconnects from one source at a time using the same method you just applied to the preamp to amp connection. Does the imbalance follow that swap? If so, the source is to blame. If not, the preamp is to blame.
3. If the preamp is the cause of the channel imbalance, then start swapping L&R channel tubes, one pair at a time. If a tube is to blame, the channel imbalance will follow the tube swap.
4. If after you've methodically swapped everything, and the process has not sniffed out the culprit, then ask for someone else's advice, 'cause this method has worked for me on several occasions, including the last time I had a bad tube, and I needed to determine which component and which tube was to blame.
1. Swap the L&R interconnects from your preamp to your amp. In other words, plug the IC coming from the preamp's L output into the amp's R input, and vice versa for the other channel. Does the channel imbalance now switch from how it was before the swap? If the problem lies with anything upstream of the amp, it will. If it doesn't, then the amp is to blame.
2. Restore the preamp to amp interconnects to normal, and then swap the L&R interconnects from one source at a time using the same method you just applied to the preamp to amp connection. Does the imbalance follow that swap? If so, the source is to blame. If not, the preamp is to blame.
3. If the preamp is the cause of the channel imbalance, then start swapping L&R channel tubes, one pair at a time. If a tube is to blame, the channel imbalance will follow the tube swap.
4. If after you've methodically swapped everything, and the process has not sniffed out the culprit, then ask for someone else's advice, 'cause this method has worked for me on several occasions, including the last time I had a bad tube, and I needed to determine which component and which tube was to blame.

