The headphone amp is quite impressive. I have burned it in for for about fifty hours or so. I sprayed all the tube contacts with DeoxIT, and I am listening to my CD player connected to it via XLR cables. I am burning lossless APE files to CDs to play, and this amp outperforms my old one by a wide margin. The clarity of individual instruments within the sound stage is impressive. I'm listening to Teena Marie's Marry Me from her Congo Square CD. The bass is tight, focused with feeling, the strings are spread out in the sound stage. The horns have a pleasant sound that blends in very nicely with the band. On top of that, Teena Marie's vocals in the mix are on point.
I am currently using Beyerdynamic DT990 600 ohm headphones. These are open single ended.
I will get the Beyerdynamic T1 second generation headphones shortly, and I already have the balanced cables for them.