Just an FYI for anyone that might run across this.
Today, while recording vocal takes, I noticed some nasty pops in the resulting takes. It took me a few hours to finally track down the issue:
The pops were integrated into the recording stream only where a tempo change was happening
Editing the tempo map to be flat (no tempo changes) in the affected area stopped the popping
The pops occurred whether or not the track monitor was engaged
When the track monitor was engaged the pops were audible in the headphone "cue"
This was not using a real cue mix, headphones were just on the Quantum 2626 phones out.
Disabling all inserts AND all instrument instances did not help
By painful process of elimination I finally discovered this was caused by the presence of Session Horns Pro (SHP) in a Kontakt instance (it was the only instrument in the instance, no special routing).
So, apparently, SHP is able to 'hear' the tempo map automation signal and, if it is changing it causes SHP to blast impulse signals to the audio stream somehow; these get printed to the recording in process. As mentioned above, disabling inserts and instruments did not help. Only physically deleting the SHP instrument from the Kontakt instance stopped the popping. And again, this was only during active tempo changes (both accelerando and ritardando) while recording audio.
To be clear, the pops were getting printed into a completely separate mono audio track, a vocal mic on the Quantum 2626, not anything on the Kontakt instance.