Usually I'm on the bleeding edge of things. Alas, life sometimes intrudes on hobbies

But now I'm finally catching up to the rest of you and wanted to report my initial operating experience with 2.9.0.19.
Installation
I installed over the top of 2.9.0.8. That didn't work so well for me. On initial install it the installer required a reboot, which was a surprise. After doing that 2.9.0.19 wouldn't start. I uninstalled using Windows add/remove, manually deleted all Thetis folders (after backing them up) including the one in %appdata% folder, then installed a second time. On the second try no reboot was required and all was well. I popped the database.xml file from 2.9.0.8 in place and it came up fine.
Faster and faster
Wisdom calc's seemed much faster. Startup is faster but not exceptionally so (this is on an i7-7700K). After working with VAC, the focus of Steve's work, I was ecstatic. I was able to finally make my software audio system perform in the manner I expected.
I have a complex, high performance, software audio system. A Presonus Studio 192 Mobile interface with a GPSDO derived wordclock is connected via ASIO to Voicemeeter Potato. So is Thetis. Potato acts like an ASIO patch panel to Reaper DAW software. Previously I've been unable to use less than a VAC buffer size of 512 (and the Presonus set to match) with portaudio set to 0/0 and ringbuffers at 0/0. Now I have the Presonus and VAC set to a buffer size of 64, portaudio 0/0, and ringbuffers 2/2. The latency is about as low as it can be.
Well done Steve!!!
VAC bugs
Note: this part was done with VAC1, ASIO, 48KHz, Voicemeeter as the device, portaudio set to 0/0, VAC1 set to enable at startup. I did not test other config's....
No pain, no gain where progress is concerned
It is possible to set yourself up for a crash when the Power button is hit after initial app launch. Under the VAC1 conditions specified above, if the ringbuffer settings are less than some critical value Thetis will crash. In my case I found that with a VAC1 buffer of 512 any ringbuffer setting less than 6/6 caused a crash. With a buffer of 64 anything less than 1/1 caused a crash. While it's perfectly acceptable to have no sound or crappy sound it shouldn't crash.
What's interesting is that if you set those numbers higher and get it started OK then you can reduce them to whatever you want afterwards. The crash only happens on the very first instantiation of VAC1.
Bug report entered on Git.
PureSignal Single Cal bugs
Repeated use of the single cal button is causing Thetis to toggle between automatic and manual (single cal) modes. Single cal should not cause a mode change or otherwise change the state of PureSignal (except to go from either disabled or auto to manual the first time it is used). It should only cause PureSignal to make a measurement and update the correction.
It's easy to see how the intent was to make single cal operation more clear to the user via changes in UI behavior, but unfortunately it confused and muddled the way PS is intended to operate. Functionally it needs to go back to the way it was in 2.9.0.8 but with improved UI indications only. I've posted a lengthy and detailed
bug report on Git