That is the quiescent current setting for the output MOSFETS. It will not affect the 40mV offset at all. Move it without either marking its position with have one of two consequences, The first is that too much quiescent current will be drawn, and the output stages will run hotter than normal. The second is that the MOSFETs will not be biased correctly, and you will get increased crossover distortion.
The 40mV DC offset is almost irrelevant IMHO. The Circuit Diagram (what our US cousins call the Schematic) for a typical Trace power amp is shown below, but I suspect your problem lies elsewhere. The fact that connecting the input to 0V or ground solves the problem means that the issue is in the preamp/EQ section. Start at the front by shorting the input to the preamp. However, the link to DIYAudio above suggests an earth loop problem, and I would not attempt that unless you are confident that you can do it safely.
The power amp Circuit Diagram linked above shows that same connections to the preamp/main PCB as the GP7. See Below: Check the ripple (AC voltage) across C29, C26,C27 and C28 as well as the DC voltages across them.