Proton 300 "The Radio" question

I have such a device, am happy with it, but the owner's manual has gone walkabout.  I see that it has a preamp output, but am unsure how to make use of it.

My problem:  I have an ailing MLAS preamp, seek to bypass it to prove that it and only it will not pass a signal.  Younger brother has a phone preamp to loan me.  If I connect it to the 'tape in' RCA, then run interconnects to my MLAS power amps from the radio's preamp outputs ... am I going to pass a signal from cartridge to phone preamp to radio tape-in, thence radio preamp-out to power amps.

Yeah, I'm unsure about the radio's preamp function. Only used it as a radio