Late answer, but maybe you did not find a solution and people want to know how to do it.
Ideally, you should have a Beomaster 6500 or 7000, instead of a 5500, because the 6500 and 7000 has pre out and pre in.
The pre out/in are connected to the Bose 901 equalizer, but you need a special phono to din cable to do this.
Actually, you can use a (cassette) tape cable with 4 x phono female connectors and a 5 pin din male connector like this:
https://www.avxperten.dk/din-til-phono-adapter/din-audiokabel-0-2m-1x-din-4x-phono-nedis.asp
You could also make your own cable, if you are handy with a solder iron.
When I had a pair of Bose 901 with a Denon amp, I used the tape in/out (play/record) to connect to the Bose 901 equalizer, and put the amp to record to get playback throught the tape circuit (like when you are recording with a cassette tape recorder on pause to "listen back" on what you are recording).
I don't know if this is possible with the Beomaster 5500: Can you put the 5500 to record and listen back to the music being recorded?