In his review, TMraven states that If your main usage is IEMs or sensitive headphones like say-- AD700-- don't get the Magni and instead get the O2. So what is the value consider as sensitive?
If you get IEMs with low impedance (usually 16 to 32 ohms) and is sensitive to voltage applied then the O2 would be a good choice since it has low gain switch 2.5x/6.5x or you can modify it to your liking with a few other combinations. The AD700 is in the same category. You should not have a problem with the HE-300. This issue only concerns you if you plan on using your setup with iems.