If you have or decide to buy passive or un-powered speakers you will necessarily need to buy a power amplifier. For the best performance and long life of the equipment, the amplifier has to match your speakers basically on two variables.
Impedance or ohms of the speaker: Speakers are usually of 4 or 8 Ohms. The power amplifier has to be capable of working at that impedance. It is usually noted in the back of the amplifier (If not check the users manual or contact the amplifier manufacturer).
Power or Watts: This is vital to avoid damaging the amplifier and also producing a low quality distorted music sound. For example, if you have a 350 Watts RMS (RMS means Root Mean Square or average power) speaker you should match with a power amplifier that has at least the same power (350 Watts RMS). At Audiodj.com we over a vast selection of the best pro audio power amplifiers like: Crown Audio, QSC and Behringer among others.