Sure, this would work -- assuming that there is no ground loop.

While not the end of the world, there is a constant noise voltage associated with the output of a power amplifier. Of course, this noise will be attenuated, along with the signal, because the High-Low converter will attenuate signal and amplifier noise, but the noise floor of the system will be elevated somewhat.

Ideally one would want to ship the phono preamp output to the main system.

I haven't read through the documentation for this unit, but in situations such as this it has been my custom to locate the phono preamp output and attach my own jacks or a direct connection. Even without documentation, especially on older equipment, this is relatively easy.

Edit: I just scanned the manual. This is a CD changer. Therefore my comment about "Phono Preamp" does not apply in this case. In my mind "jukebox" is a vintage device and equates with records.

Further Edit: The last page of the manual indicates that a stereo feed is available for external amplifiers. I don't know if this output is controlled by the unit's own Volume control.

Another Edit: Look on page 26 of the PDF file. There is a DIN socket for RECORD OUT on "CENTRALE-CD".