Ok, based on all these replies: There are 2 changes in the I/O layer, one is the new PortAudio, the other is the settings with which I'm opening things. What I'll do is first revert those settings and run a build with the old settings. If that doesn't fix things, I'll revert to the old PortAudio, at least for testing. But there are several people who had issues with the old PortAudio and not with the new one, so that's not really a long-term solution.
windows 10, 64bit, stereotool 64bit:
the issue with the new portaudio is when you "give exclusive mode applications priority" to the input of that specific soundcard (in my case, its stereo mix) using WASPI input on stereotool. MME input on stereotool works fine but introduces more latency.
this issue did not happen with the old portaudio.
all bit/samples on the soundcards and on stereotool is matched at 16Bit, 48Khz.
i can reproduce this issue and video record it if you wish.
currently the work-around for me, is to turn off "give exclusive mode applications priority" but leave "allow applications to take exclusive control of this device" enabled and also to enable "listen to this device" on a not-used SPDIF output for "stereomix", then WASPI works perfectly on stereotool, otherwise it plays in "slow motion" (timing issue).
exclusive mode reduces the latency for stereotool when using WASPI on windows.