I only now realized that we have the user & service manuals for this amplifier on site
I wasn't aware that it actually has a transistorized preamplifier
built in (circuit board 915 H 23 B, two per set). Apparently it can be
changed between dynamic (2 mV) pickup and crystal (50 mV) pickup levels
by replacing some kind of a plugin board, which contains a few resistors
and capacitors. Board 915 H 50 is for crystal pickups, do you have
those installed now?
The tape input should be the one to use with
an external amp, but it may indeed sound different (more modern, or
simply duller if you like). The internal preamp uses germanium
transistors which probably colour the sound somewhat.
EDIT: the interchangeable equalizer boards should apparently be sticking up in the left rear corner of the chassis. 915H50 for crystal pickups, 915H24 for magnetic pickups. There are also two 470k resistors at the phono input connector, they are short circuited for MM pickups.
If you have the boards for crystal pickups now, they should be easy to modify for MM use if nobody is able to supply the alternative boards for you.