The differences and improvement is quite obvious. I don't think it's only an upgrade in terms of sync to async.
The major improvement is on the headphone amp. Headphone amp on T1 MK2 is much stronger than MK1. It sounds so much better with tighter and punchier bass (comparable to Bravo Ocean) which I never expect it from Aune. Driving power is also stronger and able to drive big cans quite well now. Tested with several big cans such as Sennheiser HD600, HD650, AKG K601, Beyerdynamic DT880 and so on. It's now more suitable for pop songs and other music genre. MKI still has its advantage for classical with larger and more backward sound stage.
I love T1 MK1 but always hoping that it can have a fuller and thicker sound with stronger bass. I have it all in MK2 now.
MKI with stock tube EH6922 sounds a bit sharp on treble and dry for thinner sound headphone. But with the same stock tube, MK2 no longer having the sharp treble issue which is a big YES to me.
For sound signature wise, Aune T1 MK 1 has very huge sound stage, clean, refined, thin music body and lesser bass.
MK 2 is a big surprise to me. I would say it's a very successful sound tuning. The sound stage is not as wide as MK 1, but more dimensional. Music position slightly forward, but in return a very pleasant tube-ish sound. It's warmer with very good surrounding, details also improve a lot especially as a pure dac.