First off, the big one: do you really need to own more than one bass? It's always astounding when I travel to a summer festival to note that most upper-string players travel with just one instrument, and at most a backup bow and a spare set of strings, whereas bassists are the ones most likely to bring two instruments! But do you really need that backup?
There will be times when you definitely do. You can't assume you can borrow a bass at a travel destination or while your bass is in repair - especially if it's another player's personal instrument. Some rules of thumb when you really need to borrow a bass would be: have a sterling reputation for returning everything on time and in perfect condition; always be ready and willing to compensate - i.e., pay something; and use discretion about asking to use gear to play a gig the other person might have been called for! It's of utmost importance to bring your own "stuff": your strings of choice, your pickup, your endpin, etc. and to be certain to return any adjustments (e.g., bridge height) to the exact place you found them. More importantly, plan ahead to have your own resources. If your bass needs repair, make the appointment at such a time as the shop can make a bass available for you to rent - not everyone has 'loaners', and if they do, it may be in use. When buying up to a better bass or bow, many players are in a hurry to cash out of the old one, and sell low - if you are talking a value you can possibly afford, consider keeping it as a backup. A 'buddy system' is inestimably valuable between bassists. And, if your school's basses are insufficient for the bassists' needs, it's a great investment for you to speak up and offer to assist in the effort to acquire or repair them. Certainly, have a plan in place for a backup instrument, but prioritize using your own, so you don't sacrifice your performance (or your friends' patience). Bassists HAVE to be resourceful when travelling. Yes, we do have to drive and to have a bigger car when the violinists can carpool in a Honda. Do we need to have trunks? Pretty much. Basically, if it's your job to go somewhere and play the bass, it's your job to get yourself there with your bass. We gave up the notion of convenience when we chose the bass - using a different bass at a destination should be a very last resort.
Do you need a backup bow? In a word: yes. That's a 'don't-leave-home-without-it'. Bows are amazingly vulnerable! If there's going to be an equipment failure on stage, chances are it will be your bow.
While your bass may be fine, there are still a few things to keep in your case - I don't mean the things you already know you need to have (tuner, rag, rosin, mute, etc.), but in case of emergency:
- Things easily forgotten, such as an extra rosin and a pencil.
- Set of strings - it can happen! The nastiest 15-year-old Red Labels are better than having to go home if you break a string.
- Endpin. There seems to be no end to the parade of bassists walking around in circles looking for their endpin stuff. It goes double for those of us who use removable wood endpins. I once had a stage crew pick up my endpin when they were re-setting during a concert and put it away with the percussion mallets! Walked out on stage to play the next piece, and...no endpin. This is an ugly scene. Drill a hole through it and tie it to your bass with a string, or else keep that spare in your case!
- Battery for your active preamp (where applicable)
- Eyeglasses, if needed
- A piccolo (for those times when you're asked if you wished you played it). Okay, maybe this one is optional.
No further away than your car:
- Folding music stand
- Mini music stand lamp
- Tools: for adjusting C extension, if you have one; electric-bass adjustment tool kit, if applicable; multi-tool (Swiss army knife) if there are no specialty tools required
Things to own, in addition to the above:
- String winder, if you like, and if you have the kind of tuning machines that are compatible. Don't get me started on whether these are really necessary (no), but have it if you prefer. Just don't let me hear you say you can't change your own strings. We are bassists.
- Sound post setter. If you don't know how to use one, we do have maintenance classes at least once a year at the bass center, and this is something every string player is going to need to deal with sooner or later.
Just a bare minimum of emergency supplies - naturally, fill in with other things you know you may need. The main thing is the awareness that contingencies arise, so be as prepared as you can. The gig you save may be your own!