Intended for bassists and guitarists. Many of us are happy with their amplifiers, especially on stage (stage sound). To deliver more power they invest in a PA. Now there are two types of approaches:
1) That musician who shares the PA with the rest of the band (through a mixing console)
2) That musician who has his own power amplifier to deliver more power from his instrument
For the first scenario, the musician can connect to the power amp using an XLR cable (yes similar to the microphone cable) from his amplifier's D.I. to the mixing console.
For the second scenario, more importantly, the musician has to use the amplifier's Line Out (aka Slave Out in some amps), and not the D.I. This is because the D.I. produces a weak signal which needs some sort of pre-amplification (ex. through a mixing console) prior to feed it to a power amp. The power amp normally has a high impedance...