Is it OK to use a sub bass line with a 808 kick drum?

Yes, but make sure that the 808 kick is pitch correctly so that it is 'in key' with the track/bassline. Also, if the sub bass and the 808 are containing the same range of low frequencies, when played together you may find that you lose definition and clarity. In order to correct this try experimenting with their eq, cutting some frequencies in say the bass line may create a pocket in the 'sub' frequency range for the 808 to poke through and have its own space. Maybe try applying a slight boost to the 808's mid range (try around 2.5kHz). Even though this will usually make the 808 sound nasty and clicky when played in isolation, in context of the whole mix it can often give it a little more presence and help make it cut through the dense sub bass line. ..boom!