Actually the issue is probably with impedance. Most synths output unbalanced signals. Most pro level audio interfaced have balanced inputs. Unbalanced lines top out at -10db and balanced lines top out at +4db. What you need is a line level shifter.
It is a passive device that removes the ground hum as well as adjusts the volume leve and matches your impedance to your interface. I got one of these and it solved most of my hiss/hum problems, except one and it was a cheapo synth that was noisy from the start. Unfortunately, the only thing it doesn't fix is a noisy synth.