A good idea would be to swap EF86's to see if it follows a tube. 6922's can also account for some imbalance, but 7dB is pretty large.
In the Eros, the 75K plate load resistors are (in my opinion), most responsible for gain. Next up would be the EQ (if you have a connection issue within the EQ, it can present as gain imbalance), and lastly the tubes themselves.
-PB