i know... its a car subwoofer test. its not meant to be music. if i cranked the volume up i could probably shake the pictures off the walls with my sub :lol: and even with the headphones i was a bit uncomfortable listening to it (no, it didnt shake my brain to pieces). that is an excessive use of low end sub bass so cannot be compared with songs. i just provided it as an example that it would handle it.
the m50s and dt770pro-80 provide more low end than neutral headphones without getting some nasty distortion for highs. the m50s can be suitably refined though for music such as this
>here< but arent going to be as perfectly crystal clear as say hd598 or dt990pro's (these headphones lack substantial low end but have more clarity and better highs) but they are clear enough with decent enough highs.
besides bass drops.... i do listen to lots of j-pop, k-pop, and foreign music in general and lots of it has female vocals which require good highs. while not piercingly high, amazingly five start clear or the like the m50s certainly retain enough to sound good for such music. perhaps not as good as the hd598 or dt990pro but those lack the bass response that i desire so its okay.
there is no one perfect headphone unfortunately.
if you want a headset... perhaps look at the modmic v4 for $50. it attaches a boom mic to any headphone and is what most people use for gaming on good headphones. if that isnt your style then click "headphones" in my signature and go to the second to last post and read the "microphone reviews" ... its incomplete (i havent finished the guide yet) but there are some there to read
i dont have any of those amps so i cant tell you firsthand but only from info i've read. i use my m50s unamped since they sound fine on my cellphone and at home i normally use my 725w sound system but on occassion i do plug them into my receiver which has an amped headphone jack and they sound great. i do notice a difference between using the portable devices and the receiver but its not a huge difference to me where it sound twice as good or anything. they are a bit clearer and a bit deeper hitting on my home system.
in order of sound quality i would likely say dx < audioengine < modi/magni though mind you thats not from personally listening to all of them. it seems that you did a fair job of listing the pros for each of them. the only thing i would add is.... that the DX likely doesnt have enough juice to power headphones in the 80ohm+ range so you would need an amp if you ever upgraded from the m50s.
not sure how much of a difference you will notice on the m50s. you might have more volume control for the seperate DAC options (audioengine or mo/ma) but the DX gives you dolby headphone for virtual surround if you game. theres a decent difference from using just your onboard sound for any of the options though.
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so what should you get?
if you want virtual surround... you need to get the DX (or ST or STX) since it has dolby headphone (external dac/amps dont have dolby headphone or virtual surround). if you get headphones in the future which need a stronger amp then you could get a headphone amp and use the soundcard output.
if you dont care about virtual surround the audioengine d1 is likely a better choice for some better quality sound and more powerful amp. it also will handle some better headphones in the future (within reason... ). there may be some other choices as well but this one is commonly reviewed and listed. supposedly its also built very well.
the modi/magni is nice and definitely a good option for someone who is serious about audio quality (worth using on headphones in the $200-400 range) and wants something which will support any headphone they want. honestly for the price its top notch stuff and gets rave reviews. are there better? why sure but more costly. there are also some other similar priced items which compete as well. they get some good feedback on build quality.
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not sure if that answered your questions... but most of it is hinging on "your ears" and obviously i dont have them so i cant really know for sure what exactly is best. also i dont really know what your future plans will be or all of your preferences so all i can do is make some broad recommendations.