welcome to the site!
The manual for the 875 is
here
From what I can gather from the pics of the connections (the front has a panel I can't see very well), there are no 1/4" or XLR connections for good microphones. This looks like it's primarily a home receiver. A very good one...
Most of us use mixers that have the mic connections on it, and don't need this kind of amp. If you're wanting to do karaoke at home, I HIGHLY recommend you don't use home theater equipment, but rather, get a stand alone system where you won't trash your speakers. Most home theater speakers can't handle live mic sound for long.