Some people recommend whacking the cups/drivers as a potential *cure* for a Stax imbalance, lol.
I've heard a few significant imbalances over the years - an old L3000, RS-2, my first pair of 009S from the 1st shipping batch. You'll know when it occurs - it's not at all subtle. This is when the imbalance exceeds around 1.5 - 2 dB (sometimes a lot more). Believe me I've spent countless hours over years obsessing over "is there a slight imbalance here?" with various headphones. Mostly - nope, and you'll know for sure when you have a problem. It's not worth the heartache in the meantime.