If you want the audio spectrum artificially boosted in some way, forget the DAC2. It is neutral - which some *#"~* people describe as boring. They are probably the people driving down the High Street with bass blasting out of their cars.

I used a Benchmark DAC1 for a few years. It was neutral and super-detailed without any artificial edges but somehow it did not sound realistic. It was, nevertheless, the best DAC I could find at the time and with the right amplifier could portray a full orchestra well enough to be able to listen to each instrument individually if that is what you wanted to do.

I changed to the DAC2 because: A. it added a tiny bit of warmth B. it was neutral and naturally detailed, C. it gives a fantastic 3D sound-image, D. it streams and plays 'all' hi-res and DSD without any problems and E. It completely removes the need for a separate pre-amp.

With a good recording, an orchestra playing on my speakers now seems to extent outside the sides and behind the speakers, well into my back garden. Magically, the image also has a bit of height. Its imaging creates a superb illusion of reality. If you play a simple recording of musicians that could actually fit in your listening room, it really can sound as if they are there. When Nigel Kennedy wanders around whilst playing his violin, you can 'see' him moving. Very few systems can manage that and it was not the case with the DAC 1 nor with a dozen other DACs of all prices that I tried. The DAC2 is still the best DAC I have heard. Many expensive DACs sound like jokes in comparison, desperately trying to impress by altering the sound. Many cheaper DACs do this deliberately or accidentally. That may of course be useful for balancing out a shortcoming in your system but it leaves you with a problem if you ever want to upgrade your system.

HEADPHONE AMP
My favourite headphones are my Beyerdynamic DT 990 PRO (250OHM), which some people criticise for being a bit harsh, I'm not sure why. I don't have many dedicated headphone amps to compare it with but the DAC2 makes them sound better than anything else I have. It gives an unforced, close-up presentation of the performance with great texture, timbre and subtlety with no hint of a 'U' shape, a 'V' shape or any other shape. Dying echos, when they exist, can almost be counted. I also get a great performance using my Shure SE535s - which are again the best earphones I have heard.

For head-phone amp comparison, the next best I own is an Oppo HA2. The Oppo is good enough that it even made me sell my A&K but the DAC2 is significantly better. It seems to grab the headphones and make them do what IT wants rather than what they want. So, if you don't want 'neutral', don't buy a DAC2, but if you really want to hear the recording, I have yet to hear anything better.

I have not yet heard a DAC3 but I am told they are very similar sounding but 'technically' better.

When I first unpacked the DT990s I plugged them into a rather good Ruark radio playing on my desk. The result was an absolutely awful, screechy performance. Having recovered from the shock I remembered that I had boosted the treble on the Ruark to make speech clearer. Several reviewers say the DT990s are screechy but that really is a source problem. They do have a slight treble lift and reveal the source exceptionally well, so if the source is bright, they will let you know! 
 
Plugged into my best sources, a Benchmark DAC2 and a Beresford Caiman 2, they are absolutely fabulous. I listen mainly to classical and jazz and these allow an effortless insight to the performance with nothing masked or recessed. The soundstage is as good as I have ever heard from headphones and you can 'see' every instrument effortlessly. They are very slightly bright but that is not a problem to me and they don't cause listener fatigue. They do not make good sources screechy and they don't create exaggerated sibilance. What they do do, is remind those of us with older ears what we used to be able to hear more clearly (I can still hear up to 17 kHz but probably at a reduced level). If you have teenage ears, maybe they will sound too bright?
 
For comparison, to my ears, my Shure SE535 IEMs are the most accurate, neutral 'phones I have ever heard. The DT990's slight treble lift makes them more enjoyable and more revealing for me.
 
They are certainly not intended for portable use but.....
My AK Jr will drive them but you need the volume up to 90% and the AK Jr completely loses the subtlety that it usually delivers. Certainly not a recommended combination.
My FiiO X5's volume also needs to be around 90%. It sounds like an effort but it retains some of the quality audible with easier headphones. Again, not a recommended combination.
My FiiO X1 virtually gives up, and I don't blame it, it was silly to have tried it.
 
Good though the AK Jr and the FiiO X5 are, they are not remotely as good as the Benchmark DAC2 (nor the Caiman) and I would not dream of using a portable device at home with these headphones. If you don't have a good home headphone amp you really should try one. The difference with these excellent DT990 headphones between the FiiO X5 and the Benchmark DAC2 is enormous. You can hear the music with either portable player but with the DAC2 you can hear the performance. It is the difference between, "that's nice" and "WOW!"
 
Back to the DT990s - the comfort is pretty good. I can wear them for an hour at a time without worry but I can't say that I ever become unaware of them. Unlike closed back 'phones, lifting them slightly away from your ears makes no difference to the sound quality so a bit of careful stretching of the head-band may be all it takes to make them more comfortable. The weight is barely noticeable but I would prefer a bit of extra padding on the top of my head. I do find myself fiddling with them every few minutes.
 
Design-wise, they are functional. They don't look fashionable, they look functional. The manufacturing cost went into function rather than fashion - thank goodness.
 
Value for money - I only paid £102 and they are easily worth that.

I tried these with a Benchmark DAC1, a Fiio X3, a Fiio X1+E17 and various lesser devices including an iPhone. They seem pretty easy to drive. [size=13.3333330154419px]Music was a mixture of classical and Jazz - both CD quality and HiRes.[/size][size=13.3333330154419px] [/size]
 
My value rating is based on the £107 I actually paid from Amazon. I am having to keep reminding myself how cheap they now are in order to give them much praise though. [size=13.3333330154419px]I guess the heavy discounting of these should have told me that they were not that special.[/size]I would certainly have been pretty pis**d if I had paid full price.
 
Their best attribute is a surprisingly spacious presentation - about as good as a cheapish pair of open 'phones. To me they didn't sound any better than my £30 Jays-vJAYS except, for some reason with an iPhone. There seems to be some sort of synergy going on with the iPhone and they made it sound better than I have ever heard it. IF the iPhone was my sound source I would have been very pleased with the K550's but it isn't, so I'm not. They have a pretty laid-back relaxed presentation without any intrusive exaggeration but also without clearly presenting the fine nuances that you can always hear in real life (musicians breathing, fingers sliding on strings, valves clacking, scores being tuned, chairs squeaking etc). IF you can get them comfortable, they would probably be OK for long-term, relaxing listening but if you are trying to listen carefully, they just don't deliver.
 
Although huge, they don't actually look too silly when on your head. It is difficult not to think of Dr Who and the Cyber-men though.  They feel fairly robust and appear well made but I don't find them comfortable. I have no problem with head-squeezing or seal - that's all works just fine for my head. My problem is that the padding on the top of my head just doesn't protect me. After a few minutes I am uncomfortably aware of the weight pressing  down on the top of my skull. Easily cured with a bit of foam padding but do you really want to do that..........
 
For £107 they are pretty good value for CLOSED 'phones but in no way exceptional - unless you use an iPhone.