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

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

As you said it right, this is only on the output, not on inputs and it is not recorded at all.

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 on "main outputs" channels 1 and 2 even when the sound output 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.

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.

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

S.