I feel as though the sectioned layout is best, so I will apply that here.
 
I tested this using a pair of Beyetdynamic T90s. 
 
 
Introduction
These can be purchased for $75 + $10 shipping from eBay, $85 from Penon Audio, or around $97 from Shenzenaudio via Amazon on their website. I personally chose Penon and they were good, the processing took a little bit of time but it got here relatively quickly. The brand isn't well known, you'll find one other review of these on Aliexpress.com and that doesn't go much into detail, so this was pretty much an impulse buy for me. All I was really looking for is bass boost and high-ish output power for a good price. As far as the brand goes, they are definitely comparable to FiiO...while they aren't on the same level yet, Xduoo can definitely catch up if they try hard enough. As far as the product goes, this is a very compact device that fits in the palm of my hand yet is pretty powerful. It has a built in DAC that is 24 bit and has a sampling rate of 96KHz which is ample for most. 
 
Ergonomics
This is a little weird, I will say. At first when I opened it I thought I might have gotten a faulty product (but remembered I bought it from a store that has a small yet good reputation around here) as I tapped the device and it sounded hallow. The board is so small it takes up little room in the case, which makes me think they should have made it thinner and longer rather than fatter and shorter. Regardless, this is a small device, the pictures do not do the size justice as I thought it would be bigger, but it's very small. The body, excluding the knob, is shorter than my index finger...it's ultra compact. Other than that, the I/O is ergonomically pleasing in the way it is oriented. 9/10 
 
Build Quality
Bringing up the hallow case issue, this brings up a concern in regards to build quality, but it's nothing major. As previously stated, this is made of aluminum and it is a thicker sturdier aluminum than FiiO has on their E12 which is nice. The unit is held together with he screws, and has a face plate and back plate that can be removed to service or perhaps even replace the OPAMP...this is unclear if you can or not. 9/10
 
Input/Output
This is where it gets questionable and potentially problematic. First off, the only input you have is through USB...which is fine, but you also have no charging input so that means the USB is also used to draw power and that you might not be able to use this with a phone, and if you can it means you're going to drain your phone's battery pretty quickly...thus it has limited/no portability yet it is designed for that. This isn't a problem for me as I don't plan on bringing my open T90s anywhere and I'm just using this on a laptop, but this might not be a good choice for others. There is an RCA output, which should have been an input in my opinion, but oh well. There is also a standard 3.5mm output. As far as the knob goes, it turns very easily and if you touch it while music is playing then there will be some static. This might not be pocket friendly because of the sensitivity. 6/10
 
Sound
In general, it is quite neutral unless you turn on the bass boost which offers +3dB boost for low gain and +6dB for high gain. The gain options are +4dB and +9dB. Nothing really much more to add that won't be covered in other sections, so I'll continue there.
 
Treble
I don't pay much attention to this, but the T90s are a hot pair of cans...these do actually tame that a bit which is nice. There isn't much to add at all other than it tries for neutrality, so this won't add or remove treble unless there is too much, then it will remove a bit. 10/10
 
Mids
The mids with these are enhanced, but they're not boosted since this aims for neutrality, so not much to add here either. 10/10
 
Bass
This is where it adds things...but only if you want it, and does it give you the option to add bass. While this is no Cayin C5, this will give a slight bump depending on the gain setting (see sound for details) but it doesn't do just that. These have no special drivers, so Microsoft will give it generic sound card drivers which means it has a secondary bass boost option of up to +24dB which can means a total of +30dB can be reached. That is a massive boost, and that's all I should need to say about that. If you're a basshead, this might be for you. Personally, I got the +30dB going on without an EQ and love it. It gives a nice boom to my T90s yet preserves the mids. 10/10
 
Soundstage & Imaging
The T90s have less soundstage than the DT990s in my opinion, so I was hoping this would do something for them...and it did. While it wasn't a massive increase, it did widen the soundstage a little bit. This might be due to the fact that I would listen to these directly with my laptop's sound card and it needed more power, but it sounds better regardless. It doesn't increase imaging at all, but the neutral sound signature does help the T90s to do it's job better in regards to imaging. 10/10
 
 
Drivability/Impedance
These can pump out 330mW @ 32 ohms, which is a good amount. Addressing the generic Microsoft drivers again, it does benefit from loudness equalization so that gives a nice bump. I find that putting the Windows slider at 100% and putting the amplifier at 30% or so provides ample volume. It is capable of producing ear bleeding volume, but not unless it's on high settings. As for impedance, it can drive T90s just fine which are 250 ohms. However, the T90s are efficient, so they are easy to drive regardless. 9/10
 
Final Thoughts
I am probably forgetting something, but I can and probably will come back to update. Overall, this was well worth the $85 I spent on it. It does what I want, and it offers more features. If it wasn't for the weird I/O configuration it would be even better. There are better options, but this is a great affordable bass boosting amp, and I recommend people take a look at it and give the brand a chance. I will be enjoying this amp until I decide to fork the cash out for a tube amplifier. This, no EQ, +30dB bass boost, and my T90s = heaven. 
 
Overall rating: 9/10

Pros: Bass, clarity, transparency, imaging, comfort, 
 
Cons: Treble, build quality for the price, price

 
Introduction
I should start off by describing myself, experiences, among other things. The term "audiophile basshead" fits me well. I like bass, but also appreciate great sound quality. Give me bass and a good soundstage and I will be a happy camper. With that being said, I tend to look for open headphones that have good bass...which this definitely fits. I have only had/tried three other headphones. Those being M-Audio Q40s, Numark Electrowaves, and Beyerdynamic DT990 pros. The Q40s where my first "real" headphones other than gaming headsets that I have used for music and gaming. They were quite nice, but died within months, which lead to the Numark Electrowaves which were a replacement to the Q40s as InMusic (the parent company) didn't have any replacements for the Q40s. The Electrowaves were the same thing as the Q40s basically, except for more quality oriented which leads me to wanting quality over quantity...thus leading me to the DT990s. I had the DT990s in February ´15, but returned them within a month as they couldn't handle much bass. They did have quite a bit of bass...but a boost was out of the question which limited the bass I could get form them. After months of researching, this leads me to today and to the T90s. 
 
Burn-in so far: about 50 hours.
 
Equipment: FiiO E6 + Behringer UCA222 cheap, but will edit once I get a better amp or amp/DAC.
 
Ergonomics
These are basically the same as the DT line, except with a different design. Not much to add here other than they are stylish, yet simple. 10/10
 
Build Quality
I would say this is a significant con. For $650 (I paid $549) these have okay build quality. They have the same generic build quality as the DT line which makes no sense. For $650 I would expect more metal, but instead...there is plastic. Had the mesh been metal, I wouldn't be writing this, but it's plastic. I certainly don't think this will be falling apart or breaking any time soon, but I expected more. I could just be picky here, but I really do expect a premium feel for a premium price. 8/10
 
Comfort
These are quite comfortable, the earpads are valour and are plush as well as soft. The clamp is a little more than I like, but it's acceptable. I do find the DT990s to triumph in comfort though, the earpads on those were softer and plushier, perhaps it is because these weight a little more, but it's not a major difference. I do find that these can get a little warm though and airing out the ears every once and a while is necessary, but not a big problem. 9/10
 
Sound
Overall, this is a quality oriented pair of cans. These are not neutral by any means. They are hot, and they can be sibilant to some. The treble on the DT990s only bothered me slightly on some tracks. For these, they tend to bother me a bit more in general, but it's nothing to complain about. The biggest thing about these cans are what you put into them is what you get. They are extremely transparent, and will give you what you want, this is a massive pro as you can do what you want with the sound to make it sound a certain way. 
 
Treble
As previously stated...these are hot. The treble can be harsh to those who are sensitive to it. With a little EQ drop in the treble, this potential issue is fixed. Upon being fixed, the treble is airy, and crisp. It does exactly what it should. I cannot really go much into it as I really don't pay attention to the treble, but I would say it does treble well and that listening to it without an amp/DAC or whatever else, the treble is going to be the most present. 8.5/10
 
Mids & Details
Mids...I really don't pay much attention to these, nor am I familiar with many terms, but from what I can hear these are very present. No matter what situation I have put these cans in, the mids were definitely there, which I assume would make them "forward". As far as vocals go, this pair doesn't do as well as the DT990s, but vocals are still done very well. I think the 990s had an artificial boost to the mids that gave vocals a better sound, but I am unsure. The vocals seem very natural and as they should with the T90s. The mids are just behind the treble when just listening to them and nothing else. 9/10
 
Bass
The reason why I love these is that they can take a massive boost to bass. If you don't want a whole lot, they can do that, if you want a lot, they can do that. They can't go to an XB1000 level, but they can definitely do bass. Out of the box, they have some thump, but it depends on the song. As I said before, what you put in is what you get out, and that is definitely true for the bass. Compared to the DT990s, they have less bass overall, but can handle a lot more. There really isn't much to say here other than it can handle a lot. It really depends on what the song has in it and what you want out of them. I can say these are not basshead cans out of the box, but can be if you amplify and EQ them to act like basshead cans. Although, don't expect anywhere near the same bass levels as heavier basshead cans. Inn regards to the three bass regions, I find that sub-bass is slightly lacking, while upper bass is present and mid bass is very present accounting for most of the bass. 9/10
 
Soundstage (& Imaging)
This is where I was a little disappointed. The DT990s has a wider soundstage, but these have a deeper soundstage. Personally...I prefer a wider soundstage. That being said, these do not lack soundstage at all. They are wide to the point where they need to be, but sadly, not any more than that. There's really not much more to say in regards to soundstage. As for imaging, I am not familiar with imaging, but I can say it definitely is there. I personally think the DT990s did a better job of this, but you can definitely tell where things are with these, that's really all there is to it. 8/10
 
Leak
Open headphones leak...that is fact. In comparison to the DT990s, they leak more. I could put a more dark EQ on the 990s and put them on 50% volume and have them not leak much at all. These won't do that, but it will help. I wouldn't advise listening to these in public since they leak quite a bit, especially considering the treble is higher on these. These are definitely more open than the 990s, that is apparent. At 25% volume with just my DAC, I could hear the music from about 1 meter away. Where as, with DT990s I kind of could, but not as much. They aren't really loud and obnoxious, but people could hear and maybe make out what you're listening to. 8/10
 
Isolation
Again, these are open, so isolation won't be great on these. With my DAC, amp, and EQ on 25% volume, I could hear my TV and make out what what being said when set to a volume of 10. That is slightly louder on my TV, so the isolation is definitely not great. With that being said, don't expect isolation from these unless at volume where leak is an issue. 7/10
 
Drivability/Impedance
These are rated at 250 ohm, so that means they're harder to drive...right? Not these babies. My Note 3 was able to drive them with more than enough volume very easily. The Note 3 may have an amplifer (not sure) but they can be driven with a cheap amplifier such as an E6 like I am using. 10710
 
Final Thoughts
Overall, I greatly enjoy these headphones. There really isn't much else to add that I haven't already said. These headphones are like a little soldier. They do what they're told to do, and I think that's how the Tesla driver is designed. Beyer says they're "precision" magnets, and I definitely agree with that. They deliver the music how it's meant to be heard by default, but are more than open minded as to EQ and amplification. They lean heavily towards enjoyable rather than analytical. From metal to rap to pop to EDM to screamo there wasn't anything they couldn't make sound amazing, I am impressed. 
 
I would highly recommend giving these a try. While they are expensive and not everyone will have the pleasure of trying them (which is why I listed price as a con) they are worth every single penny I paid for them. I am extremely happy with these, and am excited to see what else Beyer can do with the Tesla drivers. 
 
Overall rating: 9.5/10
 
TL;DR - These are a very transparent pair of cans that can make anything sound amazing. They do what is asked of them without compromise. They may be expensive, but they take music to the next level.
 
Update: After about 20 hours burn-in, they seem to be getting warmer in the sub-bass and mid bass region. I'm listening to them more naturally just from a headphone jack and they sound amazing. What I liked about the DT990s was they sounded great any way you listened to them, these do take that to a new level. They are still hot, but not as much as they were. Will update again at 50 hours. I have to say though...I think I have fallen in love with Tesla drivers, the precision and efficiency is amazing. 

 
Update2: After about 50 hours burn-in, there isn't too much difference other than them being significantly less hot. The bass does seem to have tightened up a bit more too as well as is more pronounced. Listening them through my headphone jack with no EQ or anything of the sort, the treble has really tamed down. Using a vocal and treble enhancing EQ the treble still isn't bad, it was hot at first...but it's definitely more mellow now. Still enjoy them, and enjoy them much more in a natural state than I did before. 
 
Update3: I got a Samsung Galaxy S6 (early, thanks T-Mobile) and it is said to have a Wolfson WM1840 DAC chip. I was using a Behringer UCA222 before, so this is a definite upgrade...and I could see that right away. The soundstage opened up a bit and the sound got more airy. Will update when I try high quality files and with the E6. 
 
* Copied from T90 thread.