If you go with the O2, be sure to pick up a 16VAC 400mA power supply for it. (probably about $20 including shipping) This is the recommended power supply from the designer and it offers more power to the Mad Dogs than the standard 12VAC power supply that normally comes with it from JDS Labs, as an example. Also, I clipped the resistors for the 6.5x gain, effectively making my O2 gain settings of 2.5x or 1x (no gain). Feeding the O2 with a standard 2V line out, the O2 does a wonderful job with the Mad Dogs with no gain setting, especially at lower volumes. With no gain, this provides a much more useful range with the volume knob and is ideal for finding the right level to listen to with a wider range of music.
While I don't have a Magni, I do have an original Asgard, and the specs are quite similar. I like my O2 a little better in sound quality with the Mad Dogs, but it is a very close call and I am happy either way. The Asgard wins out simply on build quality and ergonomics compared to the O2. (all front-facing jacks is a bit congested) Although the Asgard is a little furnace, and I can't drag it into my bedroom and set it on my bed like I can with the O2, the Magni would not have this problem.
I actually like the fact that the O2 has front facing inputs, makes for easier switching between Inputs which I will be doing.
I guess I will buy the 130 dollar O2 now and upgrade it over time and be happy with it.
Edited by Killercoon - 6/26/13 at 3:04am