vibe slick a7 amp problem

[puntoevogp]

hi i have installed an vibe slick a7 sub in the boot as to the wiring instructions.
however am having an issue with it. when i turn the radio on the sub turns on and you can hear the sub "pop" then nothing.
if i remove the high level speaker cable from the amp and then turn it on, and plug the cable into the amp the sub works fine
anyone know why it does this?

thanks