Plug the negative terminal - of the amplifier into the negative terminal of the first speaker. If you connect the speakers in series, you're adding the speaker impedances together. Stand-alone power amplifiers amplify the signals sent from mixers to make the speakers produce sound. A stereo receiver will only play stereo two channels no matter how many speakers you hook up. Using a Yamaha PS stand-alone power amplifier with Yamaha CBR10 passive speakers is a good example to see how an amplifier should match a speaker. April 14th, by Roemtech.
There are various ways of connecting the speakers and setting up the amplifiers, which can have an impact on impedance and available power.