I do not think you want to click on "tape monitor" because that will route the "tape in" to the system. When set to "source," the tape out - in your case, your headphone preamp - should receive the output of whatever other input is selected.
The tape monitor function is really intended to use with a three-head tape deck, allowing monitoring of a signal just after it has been recorded.
The tape monitor function is really intended to use with a three-head tape deck, allowing monitoring of a signal just after it has been recorded.