Laserium typically used a Teac 3340 four channel open reel tape deck for playback prior to ADAT. The Teac's had unbalanced outputs, and so did the dBX units the audio channels ran through. The Mark 6 had 600 ohm line balancing transformers for audio (for audio mod) and data mounted on the back of the 351 card cage. Without the data return your signal would be floating and the resulting signal would be totally subject to the whims of your environment. Without the transformers the audio and data would be vulnerable to hum from ground loops.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso