I had the pleasure of tracking some bass with producer Jared Haschek on an album he's producing for Laura Jean. He sent me some early piano demos of the songs we were going to record but apart from that I had no idea what to expect going in.
In any kind of musical situation preparation helps, but being prepared doesn't always mean being well rehearsed, some times you're flying by the seat of your pants (most of my career has been built on these kind of situations actually!). Preparation sometimes means getting to know the people you're working with as soon as you can, getting an idea of their creative identities, influences, dreams and personalities. Preparation sometimes means getting enough food and sleep in to be able to function well. Preparation always means getting to a session or gig on time. In this case preparation meant turning up with multiple different instruments.
Sure, a good Fender P bass will work on almost anything and sometimes it's exactly the sound you need. That said, having other instruments on hand means that rather than massaging and processing and mangling one bass sound to work with a song you can start from the right place to begin with. (& every good finished recording starts with the best recorded sound in the first place) On top of that, any musician can respond differently and uniquely with different tools to work with.
For this session we ended up using a Fender P for an old school gospel sounding song, but the fretless 6 string worked a lot better for a soft and melodic ballad. A more aggressive part came out a lot easier on a grunty Stingray. Could we have completed the whole session with one bass? Probably, and it may have worked well. Taking 4 different basses to a session shouldn't mean you end up playing 4 different basses, but it does mean just in case that one perfect bass is proving a bit difficult on one song you don't need to settle for second best.
Having a great producer on hand to direct the session and guide the arrangements didn't hurt either. Wink.