This is a review and detailed measurements of the DarkVoice 336SE pre-amp and headphone amplifier. It is on kind loan from a member who has literally been waiting months to get it reviewed! It costs USD $310 on Amazon including free shipping. It has 42 reviews summing to 4.5 stars on Amazon so customer satisfaction must be good.This is one beast of a headphone amplifier with a massive transformer and large chassis weighing it down:Up to now we have tested hybrid tube headphone amplifiers that use tubes as the first stage but drive the headphones using transistors. That allows the output impedance to be as low as solid state amplifier yet give you the "benefits" of tubes, whatever they may be. In DarkVoice 336SE, the signal path is all tube. The configuration is OTL meaning Output-Transformer-Less. A tube amplifier needs to block high voltage DC from going out of the unit and also reduce the rather high output impedance of tubes. This is most often done with a transformer but in OTL designs, other methods are used. The benefit is supposed to be getting rid of nonlinearities of the transformer. The drawbacks depend on the design with the most common one being high output impedance.The big transformer here is for the power supply. I see no safety markings on the unit. I will be opening the unit later to see how it is inside. Until then, I would make sure to not touch the chassis any more than you have to and only use one hand if possible.Fit and finish is OK but you probably noticed the crooked power switch. The volume control is damped and a bit stiff but otherwise fine.There are a set of RCA outputs in addition to inputs so you can use the unit as a pre-amplifier if you like.Let's get into the measurements and see how she does.The owner had given me a few tubes for the first stage (6SN7). I put one in there randomly and boy, was that a mistake. There was massive amount of hum both audibly and in measurements. Indeed in measurements there were more hum than signal! So I switched to the Raytheon 6SN7 which I had from review of Schiit Saga . That reduced the hum substantially although you could start to hear it past 12:00 o'clock or so on the volume control. Dashboard measurements using RCA outs show the same:Yes, this is a total disaster as measurements go. There is really no reason for such elevated power supply hum. They reach up above the largest harmonic of the amplifier itself!Speaking of the harmonics, they fall off close to perceptual masking so they are less audible than their numbers would indicate:The Masking threshold on the right drops off exponential and so does the distortion components of the 336SE. So if you are going to have distortions, you want them this way.Switching to headphone jack, performance remains more or less the same but by loading it down with my standard 600 Ohm, distortion rises substantially:Covering our basics, here is the frequency response:Pretty good actually with just a 0.7 dB drop at 20 Hz.Warm-up showed quick stability:Problem arises in impedance measurement:Best case impedance is 78 ohm from 300 Hz up. Below that there is a large increase to 163 ohm at 20 kHz. This means two things:1) With headphones with variable impedance at low values, there will be substantial change in frequency response.2) The output is going to drop radically with lower impedance headphones.Speaking of power, let's measure that into 300 Ohm:We have plenty of power here, beating even Massdrop THX AAA 789. Reason for this is that at high impedances, power is limited by voltage. Tube amps run at high voltages easily and hence, are not limited this way. The DarkVoice 336SE delivers double the power of the THX (in single-ended mode). If we increases the output load impedance to say 600 Ohm, the Darkvoice would likely pull ahead even more.Distortion of course is massively high, reaching up to nearly 5% at max rated power! "Good news" is that it sets in gradually and there is no sharp hockey stick as we see with many transistor amplifiers.Situation degrades substantially at 33 ohm as we could easily predict:Power output is just 0.2 watts, losing massively to THX's 1.3 watts. Distortion is also sky high at 20%.Channel matching starts OK but as the volume goes lower and lower, it loses all hope:So if the hum doesn't get you with sensitive IEMs, the channel mismatch will.No sense in running more tests. We know the nature of this beast.Testing the subjective fidelity turned out to be more complicated than one would imagine. Due to high output impedance of the Darkvoice 336SE, the output level you get in your headphones will be highly variable. As such, matching levels with test tones and my analyzer got me close but not close enough. I had to match levels manually against the reference Massdrop THX AAA 789 which is a difficult trial and error. After much work I think I got some useful results but not as reliable as I wanted.Let's start with easier part of this trial, namely using low impedance headphones like my Hifiman HE-400i. Power output dropped substantially. As such the THX amp would run circles around the 336SE, producing tons more power, and dynamic range. Negating that by lowering the volume on THX 789, we were greeted with severe distortion if you turned up the volume on the 336SE. Pretty unpleasant experience. I read a bunch of reviews online and it was good to see the community's consensus being the same.What the community did recommend was high impedance headphones such as my Sennheiser HD-650. Boy, did the tables turn. I could easily outrun the the THX AAA 789 (in its single-ended mode). Because of this you could operate the 336SE at lower volumes and hence much reduced distortion. Still, push the 336SE and you were greeted with shrill highs and lack of bass. The accentuated highs did give the impression of "more space" at times and exaggerated high frequency tones on some content was a pleasant addition. Once my ears tuned into them though (through careful AB test with the THX), it became a small annoyance. With a reference to test against, if you are coming from a lower output (or no) headphone amplifier, you are going to find the 336SE a revelation. It has enough power to do justice with high impedance headphones.I then tested the two amps with my Sony MDRV6 which I think is rated at 60 ohms (?). Here, hum from the 336SE was audible when music was paused, or at times during very quiet parts. The issue was loss of resolution and impact in bass frequencies. That sense of "hi-fi" that makes you feel every note was gone with the 336SE.Switching to my sensitive AKG AK60 headphones exaggerated the situation with the Sony above. Bass impact was gone and highs become more shrill/hissy.The lesson here is that what you hear will be highly load, volume and content dependent. It will also depend on your hearing acuity as to whether you can hear small distortions and true resolution.From objective point of, the DarkVoice 336SE is absolutely horrible. It is worse than it should be but at $310 for all that you get, I guess that is to be expected.People though get such products for their subjective qualities. There, I can confirm the appeal of using the 336SE with high impedance headphones of 300 ohm and higher. There is copious amount of power here, and what distortion there is, is masked for the most part. In my experience of either power amplifiers or headphone, how much power you have available determines fidelity first and foremost. Lack of power results in anemic sound which seems to lack impact, and even resolution. The high output voltage of DarkVoice 336SE gives it such a strong advantage that it can override its much higher distortion. With that distortion following perceptual masking, it is not as much of a detriment as it may seem at first.Going to lower impedance headphones is unwise though. There is insufficient current to drive them together with large output impedance which substantially degrades/changes the frequency response of the headphone. You are much better off with a powerful and distortion-free solid state headphone amplifier such as JDS Labs Atom and Massdrop THX AAA 789. They will get louder and will be cleaner to boot.So do I recommend the DarkVoice 336SE under any conditions and lose my objectivist license with it???With high impedance headphones if you like what comes with tubes as far as maintenance, and can be blind to any safety issues here, sure, you can get the 336SE and I won't be there to hound you. Use it in other scenarios and you and I will have words!------------As always, questions, comments, recommendations, etc. are welcome.Please considerfunds using: https://www.patreon.com/audiosciencereview ), or https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054 ).