ALL open headphones will leak noise. this is why they are called open because they allow sound waves to exit instead of reverberating inside. this is why open headphones generally have large soundstages but poor bass response compared to closed designs.
closed designs are sound isolating since they do not allow the soundwaves to escape. this design often has a much smaller soundstage but much greater bass output as well.
how bad to open headphones leak?
https
/www.youtube.com/watch?v=h98XY1und0A
anyone near you will be able to hear what you are listening to even at average volumes and you will be able to hear much of what goes around near you as well. any any distance greater than 5-8 feet or so unless you have the volume cranked up you likely will not hear much of anything and certainly not through walls and doors.
you CAN still hear what is being played on closed headphones as well however it is much more muffled and limited unless listening at very high volumes due to the noise isolation properties of a closed design.
open headphones are generally fine unless you need to sit right next to someone or if you are trying to block out outside noise in which case they will not work as intended. typically its a choice you need to make depending on if sound leak is an issue or not and also what kind of sound profile you enjoy more: open & bass light or isolating & bass heavier.
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personally i own a pair of ath-m50's which are a bassy pair of closed headphones. note that i say bassy and not bass-cannons as there is a difference. the dt770 would be in the same class. bassy headphones offer more bass response than typical while still having clean sound output while bass cannons offer bass response at the detriment of all else. i like the deeper and warm sound they offer and due to the kind of music i listen and preference to have a fair bass presence they work out well for me. the fact that they are closed works for me since i can use them at work without disturbing anyone and i ignore the fact that they have limited soundstage since i know this is due to the closed design of the headphones which i require for the other reasons above.
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generally the hd558 or hd598 are a good pair of cans to start with if you want moderate bass levels that can be heard but not really felt. it isnt enough to really bring out the depth of music such as dubstep since they cannot hit sub-bass levels like other closed cans can but you can certainly listen to bass heavy music and enjoy it if you like something with some bass but not anything overpowering. adding an amplifier as suggested above can certainly give them a little more push on bass if you find them a bit light normally.
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another thing to consider is headphone size and if you have glasses or not. people with large ears or who wear glasses may want to make certain choices which fit around that aspect. i know the beyers work out excellent for people with large ears and who wear glasses but some smaller cans like the m50's i own myself arent ideal for glasses or people who have larger than average ears.
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the most important things to consider in one big list are:
-the sound profile you enjoy (do you want bass light, treble high? bass heavy treble light?)
-what kind of music you listen to (which is an extension to the above)
-what other purposes the headphones are for and how important those tasks are (such as competitive gaming which is best on bass light headphones with wide soundstages)
-if you will use them around people (is sound leakage a problem?)
-how much soundstage matters to you (which will influence open vs closed design choice)
-comfort (do you wear glasses, have large ears or have a large head?)
-amp (do you feel comfortable buying an amplifier and running extra equipment?)
-price (are they in budget)
edit:
generally i've heard akg sets tend towards bass light though since he owns a pair he may be able to better highlight what they sound like in comparision.