I don't see 'monitor' on the preamp. The 'out' of your preamp's tape loop is always active I suspect.
old days: monitor let you choose to hear either the selected source (monitor off) or the recording of the selected source (monitor in, i.e. the tape loop's 'tape in' is the monitored input)
you could listen to something, any source you were recording, by choosing it's input, i.e. hear the lp while recording it (listening to the phono), OR use monitor to instead listen to the recording, i.e. listen to the output of the recording of the LP,
Without Monitor as a choice, IF you simply choose TAPE as an Input, then you will always be listening to the tape not the source.