I've had a great experience with classical music on my 6XX paired with my Schiit Valhalla 2 (though I like my DT880 just a bit more for classical).
Soundstage is important for classical
music and the amp delivers without much in the way of coloration from the tubes.
On the format front, and especially with classical, mastering matters far more than digital format. A well conducted, well mastered piece in 192 vbr mp3 will be more enjoyable than a mediocre piece in the highest resolution. My advice would be to focus on identifying and collecting the best recordings of a given piece and don't get too caught up in the encoding format.
The Kleiber box set below is a great example, if you don't already have it. Several contenders for best recording of X piece, and in the case of his recording of Beethoven's 5th, a contender for greatest recording ever. If you have the equipment, the included Blu-ray audio version is amazing, but a 320 mp3 rip is well worth the price alone.