I think your main issue is the ground.
No synth provides a real ground lifted balanced signal. This requires a signal transformer. Active circuits can also be used to tame it.
So even if your other synths have a Balanced out, they defenetly will not have transformer isolated outputs. So its a electronically balanced, but this will not provide a proper signal isolation. The only thing it does, is if you run very long cables, 20m and more, it will reduce cable noise.
If you have 5m well shielded cable, then there is no difference in signal quality.
If you run very long wires, try it with a 1m cable and see if it still is noisy, if it is, you have an issue with something else and balanced output will not solve it.
A transformer based output might solve it, a DI box will help.
OT:
Its a very hot topic in sound studios, where they think that everything has to be balanced because otherwise the quality of the signal is reduced. But never people realy think about the way all the Hardware is manipulation the signal, they are all doing it in unbalanced way.
So you have a cable in - balanced/ audio manipulation - unbalanced/ cable out - balanced.
Only real benefit for doing it is transformer based isolation for the ground and +/- noise cancelation. But the Noise cancelation actually only works for the noise that is generated inside the cable. How real is that if you have a small studio, you have a cable that is producing noise? Go buy a new cable, it costs only 100 times less than the balancing circuit.
