What you need will depend upon the venues that you play. There have been many good siggestions by other TBers. My $0.02 worth:
For small to medium size clubs:
1) Bass guitar - I have never needed a spare bass but I do not use light gauge strings which a re more likely to break. A spare set of strings can be used in lieu of a second bass guitar. Some people change guitars to change tunings and for them a second bass guitar is a good practice.
2) Cords - Definitely bring extra cords and not a cord that has become intermittent. My setup requires two 1/4" to 1/4" cords. I brings a third cord. The minimum length should be 10 feet. Buy some of the Velcro wraps to keep the cords coiled except for what length is needed to make connections. Bring duct tape to tape down any cords that are laid down where people will walk. This avoids trips and cords being pulled out of equipment.
3) Tuner - I prefer electronic tuners that connect via a cord. Never have used a tuner that mounts on the headstock. I was given a tuner that is off frequency, so check with your other band members to ensure that all tuners use A 440 Hz as the reference. If your band has a keyboard player then you will need to compare to the keyboard(s).
4) Amplifier - A combo amp is sufficient for a stage monitor and most small to medium clubs. If the instruments are run through a PA, then any amp with an XLR output for DI use is fine. If only using a DI, then you will need some other equipment to monitor your bass playing. In-ear-monitors (IEMs)are commonly used. These are more comfortable than over-the-ear headsets. The caution is that not all IEMs reproduce low bass notes well.
For gigs without FOH support:
When instruments are not run through a PA, the major factor is whether or not the gig is indoor or outdoor. Indoor gigs can reflect bass frequencies from the walls. This can result in louder low frequencies but may also cause resonant frequencies where some notes are way louder than most others. Many bassists use a notch filter or parametric equalizer to compensate for any resonant frequencies. (At my church there is a resonant frequency for A notes. The audio tech has yet to properly compensate for this.)
For outdoor gigs it is best to run the instruments through a full-range PA. Unfortunately, some venues provide only a horn+ 12" speaker combination. Then you need an amplifier that can get seriously loud. I have used an Ampeg SVT-3 Pro and a Mesa Boogie WD-800 through an Ampeg SVT-18 cabinet for these situations. These setups have been enough. If I had known beforehand that the PA was inadequate, I would have brought a second SVT-18 cabinet for better sound dispersion.
5) Pedals, etc. - My needs are simple. A Tech 21 Bass Fly Rig provides a tuner, compression, and the adjustable graphic equalizer that I use. It also has chorus, octave, fuzz, and boost effects. Rarely are any of these effects appropriate for the songs my rock band plays. Adjusting the controls on my bass guitar and/or amplifier is sufficient.
Hopefully, some of this is of value to you.
