Bailey,
Welcome aboard.
IC is the abbreviation of Interconnect Cable, and it's the tipical RCA cable.
I agree with George when he says an out board power amp will improve the sound a lot, it happened to me with a modest priced power amp.
I'm not familiar with the design of the Marantz but on most receivers you have to use the ''Pre Outs'' to connect your receiver with the separate power amp.
Hope this helps.