Although I'm new to Earthboxes I've read enough on these forums to suggest that it's crucial to use one of the recommended potting mixes that's 70 - 80% peat based, with perlite and vermiculite.
Your compost would probably do great in a conventional planter, or mixed into garden soil, and perhaps in small amounts mixed into the peat based system or added as a 'tea' in watering an Earthbox.