The Vali has an 6.5 ohm output impedance. Any low impedance/high sensitivity headphone is going to have hiss on the Vali. The Vali really isn't suitable for the headphones you are using if you are bothered by hiss. The Magni would have been a better choice. The Vali works best with mid to high impedance heaphones. I use my Vali with the Grado 225 and there is noticeable hiss when there is no signal or with lower level sections of classical music. It's not a huge deal for me. 90% of the time I use the Vali with my DT880's. 

From personal experience that's not an issue all tube or hybrid amps have as the hybrid tube amp I have is completely silent regardless of headphone. I haven't heard the Vali but my EF2A hybrid amp(which sadly I can't find information on it's output impedance) has no hiss once so ever on any of my headphones, even on headphones as low a 8 or 16 ohms. My Magni has a noisy and hissy background compared to my EF2A.

This review was 100% about hiss, which is to be expected given the headphones used.  What did you find "interesting" about the sound?

Yeah it is all about hiss and there wasn't a mention of what was interesting about the sound. An amp shouldn't be that noisy and hissy even if there is an impedance mismatch, it sounds more like a shielding issue more than anything. I do know some tubes are more prone to noise more than others, which could explain it if the type of tubes used are known to be noisy and there likely isn't proper shielding on the amp to prevent it from picking up excess noise, which I found the case with the Magni.

 I didn't say mismatching isn't an issue, I know higher impedance headphones pick up less noise. It's because the amp is noisy. I've heard many amps with higher output impedance with low-ohm headphones and the headphones were silent.