The amp on the essence is not glorious if you have heard much better
It aims at full detail but at the sacrifice of musicality and a lot more
Also the essence stock op amps are rubbish and you should look to upgrade them to LME4720 to get a much better sound - the 650's are more than deserved of it!
The 650's will require much more than a £100 all in sound card can output to get the best from them!!
A Project Headbox II improves the sound - but really for those cans you will want a Greaham Slee Novo Headphone amp - that would be just about enough, probably the better model the Solo is more what is required.
The Essence is an excellent sound card once the op amps are upgraded and is a worthy source but I dont think the amp is any where near enough.
I think look to upgrade your op amps first - this is a cheap upgrade!
Second look to equalize your headphones - this is a free upgrade and there is instructions on how to do it on HeadFi I think - google equalizing headphones. This makes more difference than buying any upgrade!!
Then maybe look to upgrade your cans - but look to upgrade the amp as well!
The great thing about the essence is the phono outs - you can use good quality phono interconnects to get Great Sound!!!!!
EDITED - all above is based on ym very recent experience with the STX Essence sound card - apologies for nay confusion related to my posts
