Don't have much time now, class starting in 3 minutes, but here's a few impressions:
Neutrality: dead neutral, not warm, and not cold. This allows it to be spacious too, which I'll mention soon.
Lows: Not boosted, I noticed, which is welcome considering how many dac/amps (amps specifically) I've seen have a raised low end. The bass is clean here as well, very clean - headphone dependent though.
Mids: Not enough time with them, but sounds good so far.
Highs: T90 still has sharp treble, which means I don't think there's any roloff or reduced highs here.
Soundstage: HUUUUUUUGE. Best part about the X1S so far.
Clarity: very, very clear - haven't heard much gear with this clarity and spacious-ness.
Detail - above average, but not a purely analytical amp. The X1S is very musical while still bring much detail to the table.
More to come after class!