Second part first: no, there is no automatic governing by the SHD. The manual states that it’s up to the user to keep the volume at -10db or below while Dirac is enabled.
We all agree on why the -10db volume setting is there. Yes, it’s because Dirac may boost some frequencies up to 0db.
But, when 0db out of the SHD is at most 4V (requires 0db input signal which is almost never the case), and the output volume is limited to -10db, and many recording aren’t providing a strong input signal, that just isn’t enough voltage leaving the SHD. It all starts with the 4V max output from the SHD and goes down from there. If the max ouput from the SHD started at something like 8V we wouldn’t be having this conversation because I could lose some voltage from the SHD and still hit my amp clipping. Right now it’s the other way around. Preventing the SHD from clipping is holding back my amp.
It’s unfair to say “you’ve got an amp problem” when I have an amp running a higher-than-average 28.6db of gain and that puts a healthy 185w/8ohms into a pair of LS50 Metas in a very small room... and it’s often just not loud enough at -10db on the SHD. That amp can blow a hole through the side of an LS50 with the right input voltage.