Guitar amps are specifically voiced to make a guitar sound good, and Amp heads come in different power ranges and sizes. Some are very plain, some have bells and whistles, but they all must be paired with the correct 'Ohm-age' of cabinet.

Guitars require a specially voiced amplifier and cabinet in order to sound good. That's why just putting an electric guitar through a PA doesn' work. The cabinets can be different combinations of speakers and speaker sizes and most importantly will have a specific 'Ohm-age' that must be adhered to when connecting the amplifier head.