Great unit. I got one a couple of years ago, and am constantly amazed at how well it works. It simply puts to shame the DSP noise reduction in my rig, or the Timewave unit I used before. It worked great on AO-40, which was my reason for getting it. It is less necessary, but still useful, with the LEO birds, both FM and SSB. And all with one switch. I have noticed a sort of "whirring" noise when it is really reaching into a noisy, weak signal. It is just barely perceptible. About half the people I have demonstrated the unit hear it, and the others do not.
Too bad it does not seem to be available any more, as I would like to get the speaker version for mobile operations. Heil had them briefly, but for some reason seems to have discontinued them. I recall one review which something to the effect that while the hardware was rather routine, they had licensed some commercial algorithms from a major telecom. FWIW
Alan WA4SCA