I received my RU6 6 days ago and I've reached more than 50 hours of burn-in time. Here's what I think.
Before I start any further, I would like to tell you about my expectation for the RU6. The RU6 was advertised to have a reference sound, and because of that I was preparing myself for a sound that's similar or in line with the Soekris 2541, but man it couldn't be any farther from the truth. It doesn't sound like it or anything that I would classify as a reference sound.
Before burn-in and warm up
My initial impression for the RU6 was quite bad. I couldn't relate with the impressions that I found online as the RU6 sounds so unrefined. In terms of tonality, it is indeed something different. There's more subbass presence in the RU6 and an elevated bass in general. The mids is more forward, but the highs was not cohesive. It has spikes and it was messy in general. I think this is the reason why people said it was dry or harsh to them. Now, why do I call it bad? It's because of the technical qualities of the sound that was pretty much non existent. For the bass and subbass, there was no texture and seperation. It was muddled up into a jumbled mess. The positioning was also smeared all over the place. Because of this reason, I didn't post my initial impression online.
After burn-in and warm up.
After the burn-in everything improves significantly. The thing that I noticed as an immediate improvement was in the cohesiveness of the highs (upper mids and treble to be exact). It sounded way nicer and mellow to some extent. Then there's also the improvement in layering and positioning. So let me break it down one by one.
Tonality:
The RU6 had an emphasis on the lower bass and subbass region. The presentation for the mids is also more forward, but interestingly the highs is more subdued. For this reason I would call the RU6 as a warm sounding DAC.
Soundstage, layering and postioning:
The RU6 presents everything in a larger soundstage. You can hear that it extends deeper and a little bit higher when compared to LP W2 and Lotoo S1, but if I'm going to look at how the RU6 layers the sound, I would say that it doesn't do a great job in it. The instruments that are layered by the RU6 bleed to one another, giving a smeared impression for me. And the same thing happens for positioning. It was wider and deeper than W2, but the positioning is not pin point exact. It's a smeared image.
Timbre:
For me RU6 sounds more natural than the competitions. What I found really nice is the way it presents violin and trumpet (or brass instruments in general). Sometimes there's a little grittiness in the way they're presented, but not in the RU6.
Transient and decay:
For me this is the major problem for the RU6. Well depends on how you look at it. The transient was slow and the decay was long. These qualities becomes an advantage when you are listening to acoustic or classical music, but a problem to electronic music and fast beating music. For acoustic music, the long decay and soft transient made the strings sounded cohesive and better mimics the real one, but when it comes to a fast beating music, you lose the impact and the definition of the melody. When there's a snare hit, it felt like the snare was still going when it should've stopped. Hi-hats also doesn't have this sharp and definitive body, which I suspect caused by the soft transient.
Conclusion:
For me the RU6 is far from a reference sounding DAC. When someone claims something to be reference sounding, I'm expecting it to present the music as how it is, not giving anything more or taking anything away, but that's not the case for the RU6. The RU6 makes soundstage larger and it also had a soft transient and long decay. The tonality is also quite warm for my taste. If I take away the fact that the RU6 is an R-2R dongle DAC, I would say that the RU6 is a pretty good DAC. It's a natural sounding DAC, but that's about it.
Edit:
Practicality:
In terms of practicality, I don't feel like the RU6 to be the most practical one. The resistor array that is used for the volume is the major thing that hinders it from being a practical DAC. There's a slight delay when you change song or videos and the same when you are changing the volume. After a while it becomes annoying, especially when you are watching videos online or going through your playlist quickly.
Edit:
Apparently it was never advertised as reference. I must be drunk seeing it as reference somewhere.