Where did you read that from?
I looked it up on ASR and the results there was:
75 dB, 480 W 4 ohms, not 8 ohm I referred to earlier. For 8 ohms, based on the ASR review of the XLS2502:
At 440 W, it looks to me SINAD (channel 2) was about -42 dB, pretty close to what I guessed.
You can't pick and choose to suit your claim, the fact is, for such tests, 480 W 4 ohms, would be 240 W 8 ohms, based on the same output voltage, so the amp would be operating within the sweet spot, hence the -75 dB number.
As I mentioned a few times before, on relatively more technically oriented forum like this one experienced members like you and I should take the opportunity to buck the trend by not inadvertently spread hearsay, over simplified specs, measurements and their interpretation.
For example, if we all work on avoid using the wrong terms such as RMS power/watts, power consumption specs as power supply VA ratings, SINAD as THD, and using single point SINAD, or THD, THD+N measured using 1 kHz, 4 or 8 ohms as the best case scenario when it isn't, over time we will have less incorrect or misleading information on internet forums.
If there is in fact a review with measurements, on the XLS2500 that shows your quoted 75 dB, 480 W, 8 ohms then forget what I said above, but please post a link to the review.
Below is from ASR, for the XLS2502
View attachment 62058