MBdoc -
If you want to empty the steam boiler (service boiler): Do that with the steam on, at the set temperature of 135C or above (the hotter the better) via the hot water tap. If you don’t have the steam boiler on, then the steam boiler will not be drained, as the water from the steam boiler mixes in the brew boiler before going to the hot water tap. This is explained by Dave in the review/docs.
If you want to refresh the water in the brew group, then draw water from the group, with pre-infusion off preferably, otherwise you risk creating a void in the brew boiler.
Whether it’s group or hot water tap first is not really relevant. But, as already noted in the second post in this thread, I would start with the hot water tap first (steam boiler) and then do the brew boiler.