That significant difference in signal levels between the DIN radio-to-tape and tape-to-radio directions was “baked in” from the start.
As noted above, the radio-to-tape signal was effectively attenuated – maybe by 30 to 40 dB – to avoid excessive loading on AM diode detectors. In the reverse direction, the tape recorder output was fed into the “gram” input, so needed to be at about the same level as the unattenuated signal that was fed to the high impedance divider. It couldn’t be otherwise unless an additional amplifier stage was incorporated in the radio receiver, and the economics of the time were against that.
Thus directly interconnecting two tape recorders via their DIN sockets (and a reversing DIN-to-DIN lead) could be problematical, unless they had output level controls or presets that allowed the signal level to be dialled down to match the respective DIN inputs.
I think that the tape input-output section of the Murphy A674R is illustrative:
Presumably the tape out level was designed to match the radio input socket of the Murphy TR1 tape recorder (with its unlikely cascode input stage that makes one think that someone fired up the infinite improbability drive during that design exercise.)
One could say that the DIN system, at least on the radio (amplifier)-to-tape side, was used well beyond its “best by” date. By around 1970, the cost of adding emitter follower buffers to radio and amplifier tape output circuits would have been negligible, allowing them to supply “line” level signals without loading problems. Tape recorders might still have needed DIN inputs for compatibility with older equipment, though.
As a sidebar, although AM detectors were typically very load sensitive to the extent of needing resistive or active buffers for tape outputs, that was evidently not the case for at least some FM detectors. Thus, one finds some FM-only receivers (e.g. Ekco A274/A277/C274 and Hacker RV14 & RV20) where the tape outputs (which doubled as tuner outputs) were fed directly from the detectors, and so were at “line” level.
Cheers,