What a great product.
I have accrued a small collection of portable dac amps and this one is my end game. The classic and modern tube options are both great, 70/30 classic/modern usage.
Drives my Hifiman HE1000 unveiled and Prestige LTD with ease. They both sound fantastic with the RU9, especially when I plug it into a power source while I'm listening (still great battery wise, don't get it twisted).
I'm been looking to buy a dedicated desktop dac/amp with tubes. Not going to now, thanks Cayin.
Thanks Bloom Audio as always for your great customer service.

My home headphone amps consist of a solid state, hybrid, and tube amp. For portables, I'd use a btr5 and up. I've always wanted a portable that could rival my home dac amp setups while powering my favorite full sized headphones like the 600 ohm dt880 and hd600. Cayin Ru9 has made my dreams come true! Out the box, I wasn't wowed so I let it burn in and now I am in disbelief when I'm out on the streets listening to my favorite headphones without compromise! The Ru9 is end game for me at this current time for price, performance, and portability. The 3 P's. I'm hearing details I haven't noticed on the go. I get tube sound, depth in soundstage, mids are satisfying, lows are hitting, and the highs are shimmering and not fatiguing to my ears. Everything I want. Get a battery pack with magsafe and it will stick to the case. Long battery life and hyper mode on the go. No compromise. Turns into a giant brick in your pants and you'll be bricked up just from listening, you best believe it.

This thing is awesome. For years now, I've been on the hunt for a portable source that drives headphones well enough that I can enjoy them as much as more powerful desktop gear.

There have been many candidates (Fiio KA17, Hiby R4, etc.) but none of them have satisfied me.

Until I got the Cayin RU9. While it has its limits (I don't enjoy the Sennheiser HD 800S on it, for example, though that's considered a notoriously amp picky headphone by those who believe strongly in synergy), I have thoroughly enjoyed every other headphone I've used with the RU9 - including planars!

My experience has been that dynamic drivers can often sound pretty okay on portable gear, but I hadn't used a single portable device that I enjoyed something like an Arya Stealth or a Moondrop Para 2 on until the RU9.

The Modern and Classic tube modes are both great (I don't really touch the solid state mode, which sounds fine, but the tube modes are just better). I lean toward the Classic tube mode for more neutral/brighter headphones and older/rougher recordings, and the Modern tube mode for well recorded music on warmer headphones (like the Z1R).

The ability to quickly switch between the modes is awesome.

There are only a couple small downsides I've encountered: the 3.5mm connection is not as good as balanced (not unexpected for a device that's built to be balanced) and I have encountered some noise when using it.

The battery life isn't great, but I love that there's a second USB input so you can hook it up to a battery bank and not worry about that. (HyperMode, which gives you over 1W of power, is also awesome to actually get good performance from harder to drive headphones.)

The Bluetooth functionality is awesome and I've used it frequently when I need to charge my phone. LDAC stability and sound quality is great as long as your phone and the RU9 are close together.

This isn't the kind of device you can carry around the house while your phone is in another room and still get good performance. But if you think of the Bluetooth mode as an alternative to USB-C for when you want more freedom (because you're actively on your phone or need to charge your phone) then it's great, and I sort of suspect it may force the highest quality LDAC offers, which would explain why it works awesome close to the source device and not so awesome when you walk away from it.

Overall, this thing is incredible. $500 seemed like a lot for a "dongle" DAC (technically) but it's totally justified by the performance, and I've probably spent as much time on my couch enjoying music with the RU9 in the last month as I spent in the last year doing so on other devices.

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