Pretty sure the T1 mk1 and T1 mk2 share the exact same DAC chip, the Burr Brown PCM1793 which does support 88.2kHz playback on paper, however, doesn't seem to support on either version. I may be wrong, but I believe the mk2 version had a better USB receiver; the SA 9027 which allows for asynchronous playback (merely reduced jitter, which may or may not be audible)...
Looking at the new T1 mk3 (se), it looks to use the DSD1793... my initial thoughts are that it doesn't seem to be a major change. Personally, I find that DSD sounds great, but it doesn't always sound better than a redbook recording. I would take a DAC capable of resolving a higher noise floor, and not being capable of native DSD playback over a DAC that supports DSD playback with crappier specs. Granted that the cost of the T1 is fairly low; in my opinion, the tube buffer is merely "smoothing over" the parts which make the DAC inferior to more expensive offerings.
I would have preferred for Aune to try a Sabre chip implementation; something along the lines of what LH labs has been doing... these chips notoriously have problems in the treble area- the very area that a tube buffer can "fix".
Honestly, to my ears, the LH labs Geek Out v2 sounds much better than the T1 mk1 and mk2. The Geek Out v2 uses a Sabre chip for the DAC, and a class A amplifier... if you consider yourself a fan of the T1's tube buffered sound, I would highly recommend hearing the Geek Out v2. I think tube fans would also appreciate the class A sound... it's good stuff. Aune also has a "portable" class A offering, in the Aune B1, which I would consider a good pairing with the X1S --- however the price point seems silly adding them together.
I'm extremely curious as to why Aune went with the DSD1793 over a Sabre chip... IMO the 1793 chips naturally sound mid-rich and somewhat less revealing than other treble tiled or more natural sounding chips. How many of you actually listen to DSD and wish your DAC supported it natively? How many of you have compared a very good DAC which doesn't support native DSD to a cheaper DSD DAC? I have compared a bunch, and a well designed "PCM only" DAC will always sound better, from my experience and in my opinion.
I look forward to hearing the new T1 myself, but I can honestly say these days my listening is the Schiit Multibit Gungnir at home, and Geek Out v2 away from home. I have the Aune T1 away from home, with upgraded capacitors and expensive NOS tubes, but I haven't turned it on much since I heard the GO v2.
All my opinion and I welcome other opinions!