Tape loop is live all the time. It should work even with the CD selected and it is independent of all other inputs. I cannot say 100% as the manual isn't that clear.
Input selector Rotate the input selector switch to select the source component that you wish to listen to. The signal selected is also fed to the TAPE OUT sockets so that it may be recorded. Tape/MD Monitor This control lets you listen to the output signal from a tape recorder or signal processor connected to the amplifier's TAPE IN sockets. When TAPE/MD MONITOR is selected, the source component chosen by the INPUT SELECTOR continues to be routed to the TAPE OUT sockets for recording or processing.