I am more bat-eyed than bat-eared, but FWIW, here are my impressions. I hadn’t read on these DACs, so less likely to have any preconceived notions about any of them.
At the outset, I’d say the differences were not prominent, and I had to keenly listen on my reasonably neutral ER4SR earphones to start noticing them.
If I were looking for a neutral, resolving and musical (rhythmic) sound, I’d go with the RME based on the evidence in the video. I imagine it’d be the more popular choice too as most audiophiles look for these qualities more than others.
The Metrum is sweet sounding, and the voice sounds more life-like, but it suffers through some smoothening of the transients. I can imagine those preferring analogish sound and listening mostly to vocal music might like it. I’d be tempted to, but wouldn’t like to settle for the loss of definition I perceived.
The Ifi was the easiest for me to rule out, it felt strident. The Audiolab sounded like there was something awry with the timing. I wouldn’t upgrade from my Cambridge CXN to either of them.
It was the Audio GD that piqued my interest in particular. It didn’t have the low end resolution of the RME, but wasn’t fuzzy either. Somehow it felt rightly balanced to my preference and organic sounding too. It wasn’t slow like the Metrum, and had reasonable musicality. While the RME would give more insight into the music, I feel I’d enjoy listening to the Audio GD for longer hours. I liked how it sounded in my head too, which might be a function of its sound staging. If I think of upgrading my DAC, I’d consider these two options.