I It's just really to play along with songs from different sources on my Mac. .... I'm usually using a Fender & Rick Bass through an Ampeg PF500 amp. .
You can plug your Mac into the Audio In of your Ampeg PF500. Use headphones to listen to both. Simple cable.
Audio Interfaces are really about getting the audio into a computer to record, edit. If you're not doing that, then no real need for an Audio Interface. (Sure, one could say that getting better audio out of the computer could require an audio Interface, like DJs might have, but that doesn't seem to be you either...)