Amir thanks for the review and good catch on the ground. There;s no two outs about that, the design there should have been better! This is quite unusual that the Jotunheim doesn't appear to have as much power as the USB powered headphone amp.
Are you using "balanced" drive on the headphones? That's pretty important because that's what the Jotunheim is supposed to be used for. I figured you'd make sure because of the Yggy measurements where you measured it in single ended. But I was checking.
They included the single ended because they had trouble selling the original Mjonir which was balanced only outputs. But it is meant to be a balanced headphone amp. For instance at 300 ohms it puts out nearly 3 times the power in balanced drive.
This is what you should be getting. I am surprised you are getting much lower values. What do you consider clipping?
Balanced Headphone Output:
Maximum Power, 16 ohms: 7500mW RMS per channel
Maximum Power, 32 ohms: 5000mW RMS per channel
Maximum Power, 50 ohms: 3000mW RMS per channel
Maximum Power, 300 ohms: 900mW RMS per channel
Maximum Power, 600 ohms: 500mW RMS per channel
Single-Ended Headphone Output:
Maximum Power, 16 ohms: 2500mW RMS per channel
Maximum Power, 32 ohms: 1500mW RMS per channel
Maximum Power, 50 ohms: 800mW RMS per channel
Maximum Power, 300 ohms: 350mW RMS per channel
Maximum Power, 600 ohms: 175mW RMS per channel