Marzocchi told me there where no spring upgrades available for the z150? They said it has to be adjusted by air and oil only and their was only one spring available? I could be wrong but that's what Marzocchi told me when I had mine.
ya the air assit worked on my z150 when i had it. mind you one probley could use a coil out of any 6" stroke 32mm tubed marzo. next time your at your lbs take a peek in the marz parts catalogue and see whats avaliable.