Output offset on SS amplifiers - most amplifiers except tubes with output transformers - change all the time. It is a drift phenomenon, and quite normal.
Most SS amplifiers will have an offset somewhere between +100mV and -100mV. It's normal, and is controlled by the differential pair through the voltage amplifier. Once your amp is built and functioning, it will vary up and down with temperature and mains voltage.
The speaker driver is quite tolerant of up to about a quarter of a volt of offset. This is 250mV, quite a lot. What happens is that the standing current this will set up in the voice coil will very slightly displace the cone (perhaps half a millimetre) one way or the other, depending on polarity, from the resting position. This resting position is controlled by the cone surround and the coil former spider - that concentrically ring-shaped piece of impregnated cloth right at the join of the cone and the voice coil former.
The mean position of a driver cone is not particularly well controlled and usually changes during the life of the driver. Major displacements will skew the waveform, generating more second harmonic, which will sound arguably warmer, but more significantly, reduce slightly the power handling of the driver. You won't notice it.
To set the offset, there are instructions in the book. You want to set it for the lowest value at the usual time you listen to the amp. This is generally at night, around 9pm. So, measure the offset at this time, three nights running, two in the week, and one on the weekend, and take the mean. Then make adjustments to R3 to reduce offset at 9pm one week night.
And, remember, IT'S NOT CRITICAL AND WON'T AFFECT THE SONICS!!
Cheers,
Hugh
