I like the fact that you infer I have not any clue of how DEQ works from two lines I have written here
I am only describing the statements you have asserted that DEQ is what influences/requires XT32 curve to be flat. This is a wrong understanding of DEQ whether you know how DEQ works or not.
I agree that you can enable DEQ with any custom curve you design (reference, flat or with the app) and that, regardless the "initial" curve, at 0 db volume DEQ does not apply any correction.
Which is the argument that DEQ is independent of XT32 and unrelated to and not influencing any decision of XT32 as you incorrectly asserted.
At the same time, following Audyssey guidelines, that is not correct because it alters the tonal balance the way it is meant to be with DEQ.
This statement makes no sense. XT32, being independent of DEQ, makes no guidelines related to use of DEQ. XT32 attempts to output from speakers a corrected curve that compensates for room modes so that what you hear is not affected by room modes.
That is why I have said that with DEQ enabled the "starting" curve should be flat. We may of course argue about the Audyssey assumption.
This is an entirely wrong statement and you keep repeating this. There are no such Audyssey assumptions. In fact, since the default curve for XT32 is Reference, then you are NOT using a flat curve in XT32. You are trying to justify a wrong assumption that it is flat to start with!
At the same time, with a custom curve that, for instance, adds 10 db at 50 Hz, enabling DEQ o top of it will boost low frequencies too much.
If you are simply saying that two orthogonal systems can interact with each other, sure. However, the goal of DEQ is to maintain whatever sound that comes out of the speakers (with or without XT32) "audibly" be the same if you were to vary the volume.
So, it does not imply that you should use a flat curve instead or worse imply as you did before that XT32 is using a flat curve because of this.
In practice, the changes made by DEQ are very small relative to the kind of attenuations and boosts made by XT32 to correct room modes or to add room gain. So, if you really wanted some room gain for your preference. you do want the boost in XT32, not depend on DEQ.
DEQ is more like fine-tuning to account for human hearing sensitivity change. DEQ doesn't do much audibly at normal listening levels. Where it really comes into play is why loudness switches existed before. As you bring the volumes down considerably, the sensitivity of the ears change the tonal balance as perceived. Think of this as a smart loudness button. The idea is that if you liked 10db boost at normal listening levels, then at lower volumes you should perceive the same proportional boost even though your ears have lost their capacity to hear that relative boost. So an additive +12dB boost may happen and needed.