Got my Magni + Modi in today. They power my Hifiman HE-400 flawlessly, with a notable increase in quality over my Little Dot MKIII + Muse DAC or Fiio E17 with bypass. I don't notice any increase in the harshness up top like some people have said, just more definition to potentially expose flaws on poor mixes. I can feel the power out of these things. With my HE-400, I got volume pot travel to about 10 o'clock before it was too loud, so I can digitally turn the PC down slightly (using 24 bit mode) to get a bit more play out of it. The attentuators I purchased are not here yet so I cannot test them.
Got a Blue Jeans RCA cable for the Modi and I'm very pleased. If you're at all worried about buying a cable that's "too cheap," I think Blue Jeans is excellent middle ground. Not overspending, and knowing you got a high quality cable (well built) without the snake oil. Using a USB cable from monoprice. Here's some other notes...
1. I have no hiss at any volume.
2. Slight channel imbalance, but at whisper volumes only.
3. I had my music skipping every twenty seconds or so when I was plugged into the left side of my laptop, into my USB 3.0 port next to HDMI, power, etc. I switched it out to the right side, where I have a USB 2.0 port, and have not had a single skip since then. I have to assume it's due to the USB port being on its own, and not anything to do with the other one being 3.0
4. Wall wart for Magni plugged into my Belkin power strip (basic one, but very nice, $20 on Amazon ... I bought six of them) and I have no interference or anything like that.