The current sink shown should have very high linearity, since it is based on the same concept as the output stage devices. The 0.25 Ohm resistor should cause little grief (4 x 1 Ohm 1W resistors in parallel), but some experimentation may be needed here, since the base-emitter voltage of the BC549 determines the current. This circuit works by using the BC549 to steal any excess base current from the compound pair. As soon as the voltage across the 0.25 Ohm resistor exceeds 0.65V, the transistor turns on and achieves balance virtually instantly.
The 1k trimpot in the collector of the first LTP transistor allows the DC offset to be adjusted. The nominal value is around 400 ohms, but making it variable allows you to set the output DC offset to within a few mV of zero.
