The Add-On Pack extends the Microphonic Soundbox with a handful of useful items. These can be easily attached to the soundbox and removed again if necessary thanks to the included mounting parts. To say it in Carlos words: A lot more stuff to döngel around with.
The Add-On Pack V1 for the Microphonic Soundbox features 2 compression springs, 1 clockwork spring, 5 wobbly tension springs, 1 carbon strip, 2 thumb-screws and washers, 2 small magnets, ~0,6m spring steel wire.