These things show up on ebay for a starting price of $0.99 all the time, with like $16 shipping. I managed to pick up my unit for $27 shipped. Shipping was fairly quick as well, 13 days. It came packaged exactly like my little bear p5, but unlike the p5 didn't need any assembly. The specs list a 12v AC psu, but I actually received a 18v AC, same as the p5, but the wallwart was just slightly smaller. This unit doesn't suffer from cellphone/wifi router interference as noticeably as the p5 does, which drove me crazy until I figured out the source of interference. The only headphones I currently own are the AKG k240, and audio technica ath-ad700x, neither are exactly difficult to drive. But the audio technica's have noticeably more gain at the same volume setting. Both sound really great on this amp. I'm feeding the amp with the headphone output of a smsl sd-192 pro dac. I don't have a lot of headphone dac/amp experience though. I mostly prefer to listen through speakers. It definitely sounds better than the headphone amp on my EMU 0404 usb though, especially with vocals. The background is quiet, and there really aren't any real nasties. It works, and works surprisingly good. I have not tried using it as a preamp. This unit also has two NE5532 op-amps, and I may try rolling those in the future.
 
The cost of this unit is more than worth it. Rolling tubes makes an appreciable difference, and many tubes are available for cheap on ebay. Definitely a fun little device, but with limited utility, it's sized to be a portable amp. But requires AC power, and the tubes stick out. It's a fun toy, especially for rolling tubes and op-amps.
 
I didn't bother trying the tubes it came with. They're the same as the ones that came with my p5, and those sounded terrible. The tubes from my collection I've tried so far and the slyvania black plates, zenith, and rca chrome plates. Right now I'm really liking the slyvanias for flac, and tidal. But prefer the rca for my needledrops, or turntable. The RCA's do create a really low hum when the gain is turned all the way up. They do that on my p5 as well. I've still got 3 other pairs of tubes I'd like to try. But the ones I've tried so far have been satisfying.

SMSL SD-192 Pro 24Bit / 192KHz WM8805 + ES9023 Optical Coaxial Mini DAC Coaxial Decoder Black Description: SD-192 PRO is an upgraded version of SD-192 's. SD-192 PRO WM8805 done using a digital receiver circuit receives the demodulation decoding section using ES9023, reach 192Khz/24Bit. Chipset with very sound , stable and superior performance. DAC 500 yuan less than the current seemingly only SD-192 PRO with the same output level CD player with mainstream standards and have a mute function. SD-192 PRO can be connected to PCs and LCD TV set-top boxes ? ? ? HDD player game ? CD ? DVD ? MD and other devices with optical fiber or coaxial audio output . Now a large number of high-end LCD TV & set-top boxes with no analog audio output with optical fiber or coaxial output , SD-192 PRO decoded. Sound quality is very good. Specifications: Input interface ( strong signal priority, without switching ): quasi- square optical interface; Lotus (RCA) coaxial interface Output Interface: 3.5 MM headphone jack; Lotus (RCA) analog interface Power supply : AC100-240V Color: Black Housing material : aluminum Package Includes: 1 x SD-192 Pro DAC 1 x Power adapter 1 x User manual

I picked this up on ebay for $28 shipped. The description said it'd take up to a month to deliver, but i got it in 13 days. It looks exactly like the one pictured here, but says "Breeze Audio" on it. It's got a surprising amount of weight for it's size, which is about the size of a box of small cigars. Combined with some sticky rubber feet, it stays in place well considering it's small size. The binding post screw downs felt a bit flimy, but I used stackable banana plugs, so it doesn't affect me. The volume knob has no markings to show where it is, unless you count the set screw holding it in. The red led is always on, it's just dimmer when it's switched off. It's not too distracting since it's literally the size of a pin hole. The edges in the rear are a little sharp, but corner are all nicely rounded. It does spark a bit when you plug in the PSU. The bushed finish is nicely done, and even. The only cosmetic imperfection i could find was the left rear corner wasn't fitted flush.
 
I'm using it to power two energy take5.2 speakers (8omhs), and an energy 8" powered sub. I'm powering it with a creative wall wart I had laying around, it's rated at 12v 1.5a. It get's just louder than I can stand for nearfield listening. It's being fed a signal from a schiit modi. I was using a phillips AIO dvd/amp to powered the same speakers before. Right away I noticed an improvement in clarity, and tighter bass. No unwanted hiss or hums at any volume level so far. This thing shatters all expectations for an amp under $30. If this thing lasts, I see no reason to pay any more for any other no frills stereo amp. After using it as is for awhile, if all is still good, I may upgrade the PSU to see how it fares.
 
*Update - I've put maybe 10 hours on this amp so far. I've noticed on songs with a lot of quiet passages, there's a low hiss. It's not that distracting, but definitely noticeable. Real curious to see if a better PSU will help now. I'm looking at may an astron rs-12a. It's strange, but the hiss is most noticeable connecting my DAC directly to the amp. It can be reduced by using an preamp in between, and turning the amp volume up but keeping the preamp volume low.
 
Update 2: Tried a HP 18.5v, 2.5a laptop power supply. Music got a lot more oomph, especially bass. But at the cost of the some detail. Sticking with the 12v, 1.5a for now.

After going from motherboard onboard sound to external DAC (schiit modi). I noticed an improvement in clarity across the board, but vocals in particular sounded flat, and the soundstage was very narrow. I don't know if this was because I was using a DAC designed for headphones, with a speaker amp. My vinyl flac recordings were much better than my cd flac files regarding vocals. So I picked up one of these to play around with, and see if I could improve things.
 
It arrived from China in just 11 days. Pretty impressive since at time of purchase it said it could take up to a month to ship. Requires some assembly, but pretty straight forward despite lack of instructions. Mine says version 1.3 on it, not 1.2. This thing is extremely light, so you'll have to really mess with your cables to get it to sit still.
 
With the included 6j1 tubes, right away I noticed the soundstage was much improved, the sweet spot wasn't quite so narrow anymore. But the sound clarity was several steps down, especially the bass. It was very loose and muddy. I let it play for around 5 hours, and just wasn't noticing any improvement, and couldn't stand the sound anymore. So I swapped in some Tung Sol 6ak5 I had. Wow, vocals suddenly had so much more authority. The sound was really pleasing now. I can't wait to see what other tubes are like. I've got GE jan5654, Voskhod 6zh1p-ev, and Sylvania 6ak5 on the way.
 
Update: I've got about 20 hours on the amp with the Tung Sol tubes now. It's obvious this thing is adding distortion of some sort to the music. Similar to the kind of distortion going from lossless to mp3. On some types of music, in particular female vocals. It sounds really good. But on some male vocals, my subwoofer is reacting to the vocals, which sounds like when a Mic that isn't dialed in right. I really hate this thing for some songs, but love it for others. I've never played with tubes before. But I'm guessing this is just the nature of the beast. I will be trying out other tubes as I get them. I also have plans to rig up some mechanical switches that let me go from using this, to not using this on the fly.
 
Update 2: Tried some GE Jan5654 tubes. Must tighter and more detailed than the Tung Sols. Really like the sound of these tubes. Noticed I'm getting getting some random clicks and squeals with this preamp, in the right channel only. Can only hear the ticks close up, like ears inches away and music off. The squeals don't happen often, but you do notice them. I think it's emi, since I can make it happen sometimes with mouse movements and scrolling. Tried moving it around, and plugging it in elsewhere including directly into outlet. Have not been able to get rid of it. I may try to shield the case and see how that turns out.
 
Update 3: OK, this is strange. The included adapter is a 18v ac-ac. I tried a HP 18.5v, 3.5a laptop brick I had laying around on it. It not only works, but the clicks are gone, and the squeals are significantly reduced to the point they're barely audible. The tubes are glowing significantly brighter though, not sure if there will be long term damage.
 
Update 4: Tracked down source of clicks and squeals to interference from smartphone. Completely eliminated, by keeping phone away, or going into airplane mode. Currently trying out some Zenith 6ak5 tubes on it.