I have the same issue, I'm in MAC OS 10.6.6, on the latest 64-bit kernel driver..

So I  made a lot of tests,  and my results are:

It has nothing to do with electrical things or fans or monitors or amps.

This is only on the Main output, not on inputs and it is not recorded at all so that tells.

This    hiss is only present on the main outputs and not in the headphones of    main outputs,  I noticed that its volume is constant and completely    independent from the main volume knob, and it only happens in channels   1  and 2, main outputs, as a matter of fact, the hiss keeps sounding   when you start any app that uses the UX8 on  "main outputs" channels 1   and 2 even when the sound output on your app is on 3 and    4...strange.........so for now I'll avoid main outputs for everything,    its not the best solution since the main output knob and mute button  on   the front become useless, though I can use the main output  headphones   for cue monitoring on ableton live......good thing that  from the UX8 I   connect to a mixer which then go to the speakers, that  way I can  control  the UX8 Ch 3-4 volume output on the mixer, so for  now maybe  the  solution is to use the other 6 outputs.

This   result tell my logic that it is soft code related, if not it would be   present in all outputs or at least all the time and not only when the   driver is used by apps.

Don't    know if this is also present on the 32 bit version, didn't even tried    that, I saw no point in downgrading to 32-bit, so my UX8 was unused  for  2  months until this latest driver.

Are the symptoms the same on your side?

I hope this tests info helps to get this fixed on the next driver update. ASAP. please L6.

S.