I own the Sennheiser HD 650's and 800's.  I've owned the 650's for going on 3 years and originally listened to them through an Asus Xonar Essence One DAC/HP amp combo which was unable to totally lift the infamous "veil."  I eventually became so dissatisfied I sold the headphones (which I almost immediately came to regret) on ebay (sold the Asus DAC, too).  Fortunately the new headphone owner wasn't happy that the outer box had no pedigree label and returned them.  Providence?  I bought the highly recommended Bottlehead Crack headphone amp with speedball upgrade in hopes of lifting the veil, but found tube rolling highly distasteful because it was too unpredictable and the sound less than stellar so I sold the amp several weeks later.  I then bought a Musical Fidelity M1SDAC and found it lifted the veil and brought treble (life) to my music but that it's headphone jack sounded only a fraction better than the headphone jack on my old Sony 5 disc CD player.  After a ton of research I decided to expand and buy a top-of-the-line pair of headphones & a worthy amp.  I had 3 headphones on my short list, 1) Grado GS 1000e 2) Audeze LCD-XC and 3) Sennheiser HD 800.  The deciding factor was a review of the Sennheiser HD 800's with the promise of a wide soundstage and an experience akin to listening to loudspeakers on my head with music outside of my head instead of just between my ears.  I ordered the balanced CH 800 S cord with my headphones and the HDVA 600 amp for synergy between all my components.  I was tempted to order the HDVD 800 but read reviews stating the DAC side was mediocre and came to the conclusion it wouldn't be much better (and possibly no better) than the Musical Fidelity M1SDAC I already have.  I must say this combination is FANTASTIC.  When they say your music will be no better than the source you feed it they speak the truth.  When the HDVA is directly connected to my cheap 5 CD player there's quite a bit of sibilance through the HD 800's but connected to my computer via the MF1SDAC the combination brings me musical bliss.  I read a review of how much better the HD 650 was balanced so I ordered the CH 650 S balanced cord.  I'd been listening to the HD 650's unbalanced on the HDVA 600 amp and it sounded good but when I switched to balanced my jaw dropped at the huge improvement in my HD 650's.  The sound stage broadened significantly and became more accurate, detail increased substantially and the bass became tigher and more accurate.  Even my husband was impressed and he is not a critical music listener.  Although not as detailed as the HD 800's the 650's are a new beast.  I can only speak to my own experience, and the HDVA 600 is a perfect match for these 2 headphones.  All I can say is if you invest in this amplifier take the next step and run your headphones balanced.  It's worth every penny.