I've talked to them. They do not have a headphone kit for spike 2.
What they have is a subwoofer kit. To my understanding that lacks the volume control and also the option to mute the machine. For sure it is an option for me, bit I will wait until I
A. Know for sure that they will not be doing a full featured headphone kit and
B. Understand how exactly the stern kit works, a.k.a. where it connects and if it can be muted.
This patience is necessary due to an additional problem we have with the European version of the pinball machine: the coin door lacks the cut-out, which is used for the dollar bill slot in the US.
This means I anyhow have to modify any kit to fit into the hole for the tournament button.
So, in case more technical info pops up on how exactly these two kits work, please do post it