If you want to bias the opamp output stage 2mA into class A then change I2 from a 3.5mA to a 5.6mA constant current diode and change R6 from 120R to 75R. In earlier versions of the headphone amp R6 was 56R, in this case reduce it to 36R. Everything else stays the same. The class A bias is simply the difference in value between the two constant current diodes, 5.6 - 3.5=2.1mA, and the change of the R6 resistor value keeps the quiescent current the same in both output transistors. The class A bias current is drawn from the negative supply rail.

Note: Change components on "both" channels ie: there are 2 x I2, 2 x R6 etc etc.