The amp does not have an optical cable or Coax output?You mean input.
Connect your player to the Krell using either Xlr or Rca cables. Optical or coax inputs/outputs are generally used between Transports and Dac's
Personally I would say get rid of the marantz even if you were thinking using it as a transport and get a nice cdp. Or get a good transport and dac.
Balanced outputs do have some advantages (noise rejection and HIGHER output [real useful on long ic cable runs]) but is not as important as a good unit.
An in home demo is a must because depending on the room size - speakers - position of speakers and listening position can make a difference of the way it sounds. Most important will be how it sounds to YOU.
My $0.02 worth