Can't comment on the K702, but putting the Q701 pads on the K702 65AE makes them sound really really similar in sound. It would be impossible to hear a difference if you weren't carefully listening between the two.
Output impedance really shouldn't matter, what will matter is the power delivered at a specified impedance. I think the biggest difference between the Asgard and Mjolnir is in the tuning. The Asgard is tuned a bit warmer, but the Mjolnir is a really uniquely aggressive sounding amplifier. The really tight bass is a great pairing for the Audeze headphones.
They are very good units. I actually have one of the older versions of this unit I was thinking about upgrading, but I was hoping someone might have had the chance to compare the Gustard unit to one of these. With shipping and all, the Audio-GD unit comes out to be over twice as much
Thinking about jumping on one of these to upgrade my USB interface. Anyone had a chance to compare this to a Audio-GD digital interface? Anyone thinking about passing theirs along? Seems like they've hit a low price of $150 on the bay.
Here's my impression Older pads = shallow cup, smaller soundstage, more midrangeNew pads = deeper cup, better bass speed, larger soundstage, As far as material of the pads, I'd have a hard time telling them apart. They both feel memory foam-ish, and the biggest differences is the depth of the pads.
The O2 and ODAC are definitely good performers, if that's the price range you're looking at eventually. No amplifier or DAC will "add" detail, but it will reveal details in the recording you may not have heard. I don't know if the ODAC will really be a jump over the SQ of your Fiio X1 though.
Although it has a fairly low impedance it does actually take quite a bit of power to sound good. Kind of like orthodynamic headphones. I've always had a K701,K702,Q701, or K712 on hand since 2009 or so and really the best amplifier I've heard it out of is the Schiit Vali. The Vali just compliments these headphones in many ways, it's a great match. I also think the M-stage is a good choice as well, but it's not quite as sweet of a spot as the Vali.