I’ve never listened to the AAA 789. That one of course offers a balanced output, so if you’re into that sort of thing it might be nice. I’d love to compare on the basis of measurements and such, but those measurements are made at high gain, which is different for the two amps, and both their THD+N is dominated entirely by noise, which is always higher at higher gains. That said, as far as the measurements go, for $100 the Atom seems hard to beat.
EDIT - On a side note, I think I now understand what people mean about better headphones being revealing of sources. The LCD2C is sufficiently low in distortion that it allows me to notice the difference in quality between the Magni 3 and the Atom because its distortion doesn’t dominate. If I listened with some cheap, high distortion headphones, I suspect that the differences would be harder to spot.