I have noticed the same hum/machine noise/bleed, whatever you want to call it, on the Sub-outs, in addition to a background white noise level.
If you go back one output and start using Insert outs, you'll find no white noise, but the same hum. But then you lose all front panel volume control. I don't really notice any of these noises at the low levels that I monitor at, but I wonder now if the main output set at 12 o'clock is really the best option since there seems to be no hum there. I have seen some people comment on loss of dynamics from that output, but haven't heard any actual evidence of this.
Also, for what its worth, the hum seems to be related to certain controls--if you move the effects knobs for example, the noise goes away temporarily. Why it's not present on the main out tho is perplexing.
Polyphonic Analog Synthesizer