Just to be sure, I tested with another sound card (Apple dongle). The clipping that I heard at 0 dB is gone. So, indeed, it looks like that was just a problem of the first sound card (Behringer UMC204HD). IIRC, the Behringer even has the output level set to 98% on a fresh install of the driver (I didn't pay much attention to it at the time, but I remember something like that).
The rest looked the same with the Apple. That is;
shared mode has some extra peaks in the spectrum at 0dB compared to
exclusive mode. But unlike what bennetng showed in the video, for me lowering the main Foobar volume in shared mode got rid of it, just like with the ReplayGain preamp. The difference is probably because Foobar changed something since that test by bennetng. I can see that the changelog for version 1.4 says "Integration with Windows 10 Universal Volume Control.", for example. I also think Foobar no longer uses DirectSound (in W10).
I noticed another thing. I tested with 16 and 24 bit test tones and the 24 bit one produced this (Windows limiter) noise in shared mode, while the 16 bit didn't, even if it was slightly higher in amplitude. At 0 dB the 16 bit file did produce that noise too, however.
24 bit file (-0.1 dB):
16 bit file (-0.01 dB):
Notice the two peaks to the left and right of the 1k Hz signal, for example. That's how this Windows limiter noise (?) looks like for me. The above two pictures are also how the
shared vs
exclusive modes look at full volume, playing the 24 bit file.
I'm attaching the test tones too, if anyone wants to have a look.