So first things first, i am not an expert on headphones, but i am a somewhat experienced musician and music listener and i enjoy good sound quality.
I have both headphones right here and i wont list all the specs and pictures or how they are build or fit, because i don't think that is neccecary.
The Sound of the HD-681 EVO:
Warm and strong bass, tends to get a little soft and boomy.
Slightly recessed mids, still warm but not very present
Silky smooth highs , perhaps with some slight warmness to them, not too focused and sharp.
Overall the EVO is more a "listeners" Headphone then a "reference-type", it provides a lot of smoothness and warmth, but still has good clarity and separation between instruments.
I would recommend it for Blues, Jazz and generally more soft and relaxed music.
The Sound of the HD-330:
The Bass is tighter then the EVO and has a good amount of punch, but it doesnt sound bone dry and still is warm enough to listen effortlessly to it.
The mids are more pronounced and have small peak at about 2khz, but i need to do more testing to fully confirm it. very transparent and neutral sound
On the top end you have slightly more crisp and glassy highs, but they are not point-sharp and just right. still a lot of transparency.
Good for Rock, Metal and Electronic Music, generelly better for faster and agressive music, also good for gaming.
Overall the HD-330 is the far more ballanced and neutral headphone and it seems to have a better resolution then the EVO. The Difference in volume isnt too big between those 2 and overall they both are a great bang for the buck.
i hope i could give you a small insight in those 2 headphones and excuse me for my not-so-flawless english (im from germany) :)