my very first processing element in the macro is
Process nomenclature here. The Macro (or chain, or batch) is a single instruction that calls a collection of tools that naturally go together. That’s what Mastering Macro is.
The Mastering Suite is all the stuff you do manually to get between your raw reading and the submitted sound file. In your case, the Suite would be High Pass Filter > Mastering Macro > Noise Reduction.
In the bad old days, before the Macro, you used to have to execute the three mastering tools individually, one at a time.
That gave you a Suite like this.
High Pass Filter > Filter Curve > Loudness Normalization > Peak Limiter > Noise Reduction.
Let’s zoom into those first two tools. High Pass Filter’s job is to suppress very low pitch sounds, microphone errors, thunder, rumble, and earthquakes. Filter Curve’s job is to suppress very low pitch sounds, microphone errors, thunder, rumble, and earthquakes without, as much as possible, affecting your voice.
So in my opinion you should not be able to tell any difference by leaving out the High Pass Filter. That effect is built into the Macro.
You did do one thing that can cause problems. ACX puts great stress on matching chapters. There is a soft, fuzzy rule that once you settle on a Mastering Suite, you should continue with that exact Suite through the whole book.
Only applying Noise Reduction to some chapters makes me nervous. Are you using Noise Reduction of the Beast (6, 6, 6)? That was chosen because it does effective, gentle reduction that nobody can hear working. If you use other numbers, you can get voice changes between chapters.
I know we’re doing all this after the fact, but those are the concepts.
See if you can tell any sound difference between the High Pass Filter and not. My guess would be to drop it as a hot rock. If there is a difference, the voice without it should be more natural.