I bought the ZO a little while ago. The headphones I am using are the Mad Dogs by Mr Speakers. My source is an iPod Touch 5th gen.
First of all, the included interconnect is complete garbage. It started messing up after a week.
When I tried to power these 50ohm headphones with the ZO, it wasn't enough. I didn't use an LOD cable, I just double amped it. In high gain (pink) mode, it sounded horrible at high volumes. It was distorted, but even so I didn't find it very loud at all.
What they did work with was my apple earpods. When you pair this thing in high gain mode with those earpods, they sound fantastic.
So a few days ago, I got an Objective 2 amp. I bought it because the Mad Dogs need more power than a ZO2.
Just this evening, I bought some new interconnects so I could try triple amping the setup. (iPod to ZO2 to O2)
I was able to get the volumes I wanted, but the distortion was still there in pink mode. Even though I was using a 3.5mm interconnect and not an LOD, I decided to try low gain (purple) mode.
In this mode, the distortion disappeared completely. I was about to sell this thing off, but apparently when you use it in low gain mode in a triple amped setup it works perfectly. I got exactly what I wanted.
Basically, this thing seems like it works very well with IEM's, but with full size headphones, you should expect it to give you more bass, but not the volume you want. For both volume and bass, you need another amp to supply the power.
I could be wrong about different headphones with lower impedances, but with the Mad Dogs, you need power.
Edit: My mistake, you're not supposed to raise the volume on the source too loud. The distortion disappeared once I reduced the volume on the iPod in High gain mode. It works fine in pink now, all I had to do was put my amp to full volume.
Edited by Goofy145 - 1/9/13 at 11:10pm