Isn't it funny how three people can give essentially the same answer to one question, & manage to make it look like three different answers?
My profuse apologies for contributing to the general confusion.
I hope this is clearer:
There are three options —
• If you just want to HEAR (but not record) your keyboard playing the bassline through the studio speakers, then all you need to do is plug the keyboard audio outputs into your interface (Firebox?) instead of your keyboard amp. Then that means that the audio from your keyboard can be monitored in Logic (or possibly directly, bypassing Logic, using Firebox software). This is what BB said. The reason you can't hear it with the keyboard audio leads plugged into the keyboard amp is that what Logic has recorded so far is not audio, but MIDI; & MIDI is just data for producing sounds from instruments: it doesn't contain sounds in itself.
• If you want to hear AND RECORD your keyboard, then (again, as BB said) set it all up as above but with Logic set to record your bassline as audio on an Audio Track (probably mono, but just possibly stereo). Once you've recorded the audio, you will have two recordings of your bassline; one as audio (freshly recorded) & also the previously-recorded MIDI track. There doesn't seem to be an obvious reason for having both of these; everything will eventually end up as audio when it's been mixed, but OTOH the MIDI track has the advantage of being easy to edit & to chose different bassline sounds later if you should want to.
• If you don't particularly want your keyboard to provide the sound, but would be happy for software to do so instead, then you don't need to do all the setting up above. All you need to do is take the track you've already recorded your bassline onto, & assign it to an Audio Instrument (e.g. the EXS24 Sampler).
