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....
The "C" is the "common". That is the black wire or the - the other 2-4-6 ohm connection is the red. also Make sure you match the correct ohm connecton.

red is always positive.
C means "common" or ground. (negative)
The two blacks are negative (c stands for "common", sometimes also called "ground") and should be connected to the - on your loudspeakers. The red ones are positive and should correspond to the + on your loudspeakers.