This is a simple guide to the various methods of using a DI box and playing directly through a PA.
If your amp doesn't have a DI out connector but does have a Send/Return loop that you are not using then you can try connecting a jack lead to the Send connector then put that into a DI box to convert it to a balanced XLR signal. The signal from the DI box can then go to the PA.
Connect the balanced out straight to the PA.
Usually there will be another output that you can also connect to your amp or a stage monitor so that you can hear yourself. For best results from your amp you want to bypass the preamp, if possible. There are different ways to do this on different amps - if there's a “power amp in” connector then use that. Alternatively connect to the Return of a Send/Return loop. If you have to connect to the normal bass-in jack then turn the gain down low initially and set the tone controls flat so that they don't affect the sound too much (the amp simulator has already shaped the sound).
As always, when connecting any two pieces of equipment for the first time, keep all of the levels (volumes and gains) down low to start with and bring them up slowly to get the correct level. This avoids damage to equipment and ears.
Phantom power can potentially cause problems when using method 3 above. Phantom power is a voltage (usually 48V) sent down the XLR connectors of a mixer to power condenser mics attached to the XLR channels. It does not affect dynamic mics (e.g. bog-standard Shure types etc.). It should not damage pre-amps also attached to the XLR connectors of the PA but, in some instances, it can. Damage normally occurs if one of the three lines on the XLR cable is intermittent or not working, so ensure you're using decent quality, undamaged cables.
To avoid any possible damage to your amp/pre-amp when connecting it to the PA, you can insert a DI bix between the pre-amp and the mixer. This will stop the phantom voltage from ever reaching the amp/pre-amp thus avoiding the problem. Having a DI-box in your kit bag will also give you a backup should the amp/pre-amp ever fail as you can use this to connect you bass to the PA directly.
Types of DI boxes vary enormously. The simplest ones will have only inputs and outputs. Others are available with attenuation “pads” (to reduce the signal level) and various linking options. The most versatile ones have tone shaping circuitry (and act as mini preamps in their own right).
