Not stated but inferred is that when switching the two on together, or after the pre-amp has been on only a short time, there is no audible thump. This suggests to me that there is an abundance of energy built up on the pre-amp output during use that is discharging through the power-amp when you turn on the latter. It would appear that the solution is to power down the pre-amp prior to using the power-amp, and to power them up together when it is desired to use the power-amp. I don't know how modifying the switch on either the pre- or power-amp would alleviate the problem, unless it is paralleled with the power-amp switch to ensure they always power-up together. In that case, plugging them both into a switched power strip should do the trick.