R-390(non A) does not have the audio output power level of
SP-600. It has 0.5 W vs 2W. The LC filters in wide IF position must
be very similar- 16Kc and 13Kc (slightly more in Collins).
Just for fun I connected Sony 7600 to a Hi-fi stereo not long ago.
Sounded pretty decent for a portable on SW (especially with the synch
on). Did synchronous detectors exist in the
fifties?