I put a ruler on my screen to try to look for the imbalance and I wasn't able to see it there. I think you should start by playing a 60Hz tone into the Eros with a different source and measuring the AC voltage at the output jacks. This is a little hard with step-ups since the typical sources available like old phones and tablets with 1/8" TRS jacks will tend to put out a little too much signal at lower levels, so you may have to get a bit creative on that. This is an important step, however, as you'd want to narrow things down to the Eros itself.
Other tests to do would be to swap the incoming channels and recheck, swap the outgoing cables and recheck, then swap the EF86s and recheck.