5 watts is pretty loud for a sweep.
I do have the frequency response for a range of speaker outputs, though. If the amplifier was changing, other than output level, then so would the speaker.
I don't see it in these 24 sweeps:
View attachment 271794
Ambient room noise interferes with the low level low frequencies, and my room has a hole around 48Hz.
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As for phase, the same situation.
Here, the three lowest level traces are removed as they didn't fit the pattern of the other 21 sweeps, also, likely noise related:
View attachment 271796
Distortion:
Measured distortion is dominated by noise until the the highest SPLs are reached, where the distortion measurement bottoms out then begins to rise a little.
Lowest SPL at the top, increasing toward the bottom trace.
The bold trace is the highest SPL, and measured distortion has just begun to rise out of the noise floor,
View attachment 271802
Conclusion:
No audible differences as the sweep levels are increased from very low to rather high. Hearing protection desired at the higher sweep levels.