You can always put an inline resistor in the "+" lead of the channel with more output. To reduce the gain by about 2dB, just buy a few 1/4 watt resistors (make sure they're non inductive). If it's really about a 2dB error across the spectrum, adding an inline resistor between 8.2 and 15 ohm should do the trick. If your driver is exactly 50 ohms a 12 ohm resistor should do as a first test, and adjust and replace as needed. I've found the Fostex drivers can vary, I've seen 48 and 57 ohms, so a few trials may be needed (especially if your 2dBis a swag).
A more accurate way (assuming you have a multimeter) is to get a variable resistor and trim it until it sounds balanced, then measure the resistor and replace it with a quality non inductive network of resistors giving the same value...