[diyaudioguy]

I had the same issue with too much gain for my headphones (Sennheiser HD598) using my EF2A.  The voltage gain for the amp is provided by a Burr Brown OPA2604AU op amp.  I changed a resistor in the feedback loop from 6.8K to 15K to reduce the gain.  Fortunately this is a through hole standard 1/4 watt resistor.  Taking the EF2A apart requires some care.  First remove the tubes, the nut on the headphone jack and pull the volume knob off.  Take the front screws off of the case and remove the face plates.  The potentiometer is attached by a ribbon cable so use some care with this part.  Remove the four screws from the back and the whole circuit board slides out of the case (attached to the back by a voltage regulator and the RCA jacks.  The Phillips screw on the back is for the voltage regulator (for the tube heater supply) and the pins of the regulator are soldered on the board so do not remove this screw and be very careful not to break the regulator's pins.   The op amp is located at the front of the board (between the headphone jack and volume control).  On either side of the op amp are two of the resistors in the feed back loop (that ones that connect from the - inputs to ground) which were 6.8K 1%.  I de-soldered these resistors and installed two 15K 1% metal film resistors (this is what I had on hand) which cut the overall gain of the amp in half.  The normal listening position is now about 9-o'clock which eliminated the problem I had with channel balance.