There's not much at all to using the A-Designs ATTY as once you make your connections you're more or less good to go. It only has a volume control knob and mute button as controls, which is definitely self explanatory. I think the main thing with the ATTY is setting it up and finding a proper usage for it. I haven't ever looked at the manual for this and don't expect to be needing to.
The A-Designs ATTY is a passive box and won't affect your sound quality positively or negatively. It's simply a line level controller so sound quality isn't really a consideration here. I will say that I haven't noticed any decline in quality when this box is being used, as it seems theory is correct here with this being passive...
While not everyone is going to have a need for A-Designs ATTY, for those who do this is a high quality option at a pretty reasonable price. A-Designs outlines all the uses for this on their site, but I've used it for volume control on a passive set of speakers for the most part, although it does float around at the studio and will sometimes be used for other purposes. It's something that's going to be needed as a necessity or utility and not as a luxury or tone item. Again, if you have a need for something like this, ATTY is one of the only of it's kind that I've seen and is one definitely worth exploring.