Mcintosh Speaker cable connection question??

When I connect my speaker cables to my Mcintosh Amp, how do I know which connection is positive & which is negative, it doesn't show a + or - sign n the amp. It has 2 black plug-in with a (c) above it, and 3 red plug ins for each of the 2 channels. The red ones identify 2,4,6 ohms which I understand that part. My speakers do have a + & - on them. Please help....
Thanks...