A combo amplifier contains both the amplifier head and speaker cabinet in one intergrated unit.

When it comes to Amplifiers, it's all about the voicing. Guitar amps are voiced especially for Guitars, which is why an Electric guitar just DI'd into a PA doesn't sound so good. Keyboards, need a specially voiced amplifier too,as they emit a much broader spectrum of frequencies. Here are the amplifiers specially designed to sound great when used with keyboards or synths.

Acoustic guitar amplifiers are specifically designed to produce a clean, transparent, acoustic sound. They will usually have a relatively flat frequency response so as not to "colour" the sound too much.