Hello, new user here, recently inherited my mother's like new beogram 1700.

I have had this unit for 2 years and it has been a trusty friend with 0 issues...until I attempted to update my speakers...

Until Christmas, this beogram was connected to a similar aged receiver and passive speakers.  Things ran perfectly smoothly.....but life is changing and I needed to downsize my set up.  So, I purchased a behringer pre-amp and active Edifier speakers.   Suddenly, sound only plays from one speaker at a time.  I can swap which speaker plays by switching the wires coming out of the turntable.  This happens when I bypass the preamp entirely, only sound from one speaker that I can swap by switching the turntable wires (albeit very quiet sound).  The other speaker just hums.  I assumed this was a cartridge or wiring issue with the beogram, so I brought it to the local repair man and he said he tested the beogram and it produces sound strongly from both channels.  He even tested the preamp.  He swears the beogram is in perfect condition and has no clue what is going awry.  The speakers are brand new and work perfectly well with phones and computer input.  

Any help, please? Is the local repair guy just wrong?  Is there some issue with translating between vintage B&O technology and new RCA input active speakers?  Thank you so much, friends.