I’ll try and keep this short and only talk about the important things in an abbreviated form (as best as I could given the word count I ended at) but the RU9 is a product that I both really like and dislike in certain ways. The overall package with this device at $499 is fantastic and I am satisfied with this purchase but there are issues that hold it back and is why I gave it the score I did even though I really love the sound coming from it.

I wont talk too much on the sound but it was incredible on all three available modes and was immediately an improvement over the Qudelix 5k I use at work. Testing was done with Hexa’s, Monarch MKIV, and Focal Clears. Classic tube mode ended up being my favorite but found myself switching between Modern and Classic depending on my mood. The solid state mode was nice and neutral sounding but I don’t really go for it as the main point of this thing is to listen to one of the tube modes in my opinion and is really what gives this thing the value it has and was pretty stable with no tube ring unless I was really trying to get it to come out.

Battery life on the RU9 in my experience sits somewhere at a bit over 3 hours with Balanced input and closer to 3.5 hours on the 3.5 mm all done on Tubes. I was able to get closer to 4 and 4.5 respectively on the solid state mode. This is definitely a bit shorter than most would like but with my case I’m lucky to usually be not too far away from an outlet. Using the usb + power mode available in the setting gives really great battery life at the cost of your phone battery. Another option is to use a magsafe battery such as the INIU which also gives access to hyper mode which is probably the best option and is what I do.

The magsafe feature really helps with the transportability of it and paired with a relatively thin phone gives the RU9 a great pocket feel when heading out on a walk. However, on extended listening experiences I found myself using the magsafe to attach to a portable battery bank rather than the phone. Adding this all together in a pocket does seem like much even with looser clothing so after about a week or two I found I’d much rather reach for a TWS in these cases if I needed an extended listening experience on the go.

Bluetooth in my opinion could have probably just been left out if it cut the cost. The range is pretty terrible and just walking from one end of the room to the other end was enough for the RU9 to drop the connection. The only time I use the bluetooth mode is for resolving issues I had with the RU9 which the rest of this review talks on.

Now the problems I have with this thing come from absolutely terrible crackling I experienced taking this thing to work (The reason I purchased this in the first place). The RU9 seems to be extremely susceptible to EMI so I had to go through multiple cables and leave it at a specific spot on my desk to get the crackling down to tolerable levels. This makes it pretty much unusable at work unless I put the RU9 on bluetooth to keep it away from both my phone and desk which eliminates it entirely. Using this thing at home though is great and I don't have any of the major crackling problems that I do at work but was not really why I purchased this in the first place since I already have an at home setup.

When in my pocket I also occasionally notice crackling but within tolerable ranges and suspect it has to do with the antenna position in my Zfold7 but I’ve learned to deal with it. It’s honestly disappointing but I assume people using a DAP or phones with better antenna positioning wont have this problem at all.

Cayin RU9 | Vacuum Tube Portable DAC and Amp

This product is absolutely spectacular and as far as portable amps go it does a phenomenal job with my headphones. There was a factory defect with my first pair and Bloom did an exceptional job of replacing it, and I have never been happier with an audio product..

Coming from the orginal ifi go blu paired with my hd6xx this is a game changer.
I listen almost exclusively in bluetooth mode, as LDAC sounds as good as a wired connection to me and I like feeling untethered.
I always had a feeling I am not getting the most dynamics out of my hd6xx with the ifi go blu although it does reach satisfying SPL levels.
By contrast, the Cayin RU9 can push 4 times as much power through the hd6xx drivers in hyper mode (minimum 20watt usb port). I was pleasantly surprised with the greater flexibility and ease I was granted in eq'ing the hd6xx to my preference without reaching the limit of distortion. Having greater flexibility in tonal control with the classic, modern, and class A modes is nice, though I tend to leave it in classic mode.
I recall tube rolling genelex gold lions into an old little dot amp, and the gooey tube character was very apparent and charming, if syrupy. The Cayin RU9 does not reach the full-fledged gooey tube nature like that amp. But it does introduce just enough tube character to keep things interesting, without generating significant heat like traditional tubes.
If I had to detract anything from this device, it's that I am not finding great use for the magsafe compatibility, bluetooth range is not as good as the ifi go blu, and that the battery life could be better. In terms of battery life, best case is I can get 4 hours in medium gain listening at medium to high levels. At the same time, I don't leave the house with this dac so I am always near a power source. To me, the Cayin RU9 is portable in the sense that I can take it from the living room to the dining room.
My suggestion for those considering this is you have two options if sleekness, power, and bluetooth are your priority, this or the ifi gryphon which has equivalent power output into 300ohm, without necessitating auxillary power. I wanted a departure from ifi, and I was curious about how Cayin implemented the nutubes into this hybrid dac. For my use case, 5/5

This is a pretty unique piece of equipment. Between the tubes and hybrid mode, as well as the magnetic features, this little amp/DAC is kind of a category of its own.

As the reviews point out, you do get three different sound signature qualities. A warmer tube sound, a more neutral solid state sound, and a hybrid mix in between. The size of the unit is very portable size wise, and with the magnetic back, this little unit will stick to your phone and not move out of place. It's game changing. I'm using this with a pixel 9 pro, but with a peak design everyday case which adds the magnetic feature.

There is one one negative, and that is the battery life is not very long. 3 to 4 hours maybe. But, there are some really affordable magnetic back batteries on Amazon that have been suggested in some audio file communities. The Lisen Brand 5000 mAh battery on Amazon works seamlessly with this. Yes now your rig is a little thicker, but you have double the battery life. Not always needed though. So that kind of helps that negative not hit so hard.

Fantastic sound, adjustable on three modes, which I change depending on genre sometimes - and they each can make the monarch Mark IV really sing.

The second negative is the Bluetooth antenna is extremely weak. Your phone has to be very close for this to even work. I mean it's good the feature is there but they really should have engineered it better. Using a USBC Bluetooth dongle which gives you other codecs, the reception is slightly better. Enough to walk around with the whole setup. But ultimately it's still not ideal.

Five stars if they give it a little bit more battery life and fix the power of the Bluetooth transmission. But even with these negatives it's still four stars because of what it does overall.