Thanks Christian.
I followed the first part of this solution (removing Gearbox) and then simply downloaded the latest version of Gearbox, connected my device, updated drivers & firmware, and Hey Presto; Garageband sees my X3 Live no problems!
Thanks for your help and comprehensive guide to get this sorted out.
My setup is a circa 2011 MacBook Pro 15", with 8Gb memory.
Regards,
Nathan