Im going to start with I'm no audiophile but I do enjoy listening to music and I do like these headphones. I have, and very much love my Sennheisser HD 558s so my opinions may not be 100% fair, as they are a totally different.
I treated myself to a new gaming pc a while back and wanted some open back, super comfy headphones so I bought the 558s which Im in love with. The only thing is my wife wont let me watch Netflix in the bedroom with them as they leak sound. My many pares / styles of cheap ear buds after a while really start to ache night after night, so I went on the look for something discreet, comfy and good sounding. In which I spent too long looking before I settled on buying the 451's. I tried a load of headphones in music shops but found it hard to not compare with what I already own. comfort and quality for the price.
On first impressions the AKGs are very light but solid feeling but looked cheaper than the £50 tag because of the crazy thin cable (marginally thicker than standard ear buds). I like the fact the cable is detachable and easily replaced. I have replaced mine as I felt the cable is very thin and detracted majorly from there looks(sound has not changed for what I can tell.) As silly as it sounds I'm now very happy with the build quality and looks with the £10 change, and are well worth £60. The original would only take a few running's over with the computer chair.
There are plenty of mixed reviews on the 451's and I can see why after buying them...I hated them to start with and I felt cheated. I read that these where greatly improved with "burn in" . So I set mine off for no less than 72 hrs using my mp3 player on shuffle loop, checking them morning evening and night in hope...with no change what so ever. I was contemplating sending them back with my other niggle being the cable.
What im saying is in my opinion "burn in" does not work on the 451's but are totally totally transformed with a slight tweak in the eq settings. Im no expert and don't pretend to be but I know the sound I like and don't like. and by lowering the middle eq settings cleaned up the sound and made them more neutral and less "bright" sounding which originally made me almost wince. I listen to a lot of "alternative" music, I'm an easy listener but generally steer away from pop & dance music. I have my entire cd collection as flacs on pc to scale to mp3 320cbr for portability.
The 451s are comfy up to 2 or so hrs until I get that anxious feeling (I find myself with the 558s music has stopped but im still wearing them for some time after until I realise) but not with the akg's. I have had them for a few moths but would be happy using them on a plane for the sound quality, portability (tidy hard case is great) and size.
I listen to music mainly on the pc and occasionally around the house with the 558s but now reach for the akgs for house and gardening etc. on the move great but sat still for any great time I find the comfort can detract slightly from the "escape" though music. I do get the sense of claustrophobia easily but the akgs are very mild compared to a lot I tried on for comfort in the shops
My wife does not complain about them leaking sound at a decent volume so I can now watch Netflics and tunes in comfort and peace in bed and around the house. They don't isolate the outside sound a very well without music playing but with they are pretty good even with tunes at a low volume.
Personally I'm very very happy with the build (with the cable change, sound quality (with the eq tweak) especially with the size as they really do pack a good sound into them. They are discreet unlike some and comfy and really would use them on the street without feeling self conscious.
I do believe you can read too much into reviews and waste lots of time comparing things. just think If you have read a few reviews on the 451s you could have probably spent more ££ on the next level up headphones using your hourly wage.
Buy, tweak, forget and enjoy!
Hope my opinion helps at least one person trying to choose a high performing set of headphones for a modest amount of hard earned cash.