Must be the topology used then and the way the tubes are biased, type of tubes, how the power supply is configured...too many variables. Tubes do have more distortion, and will only increase when things get louder, so this might be it.
To build on what @Azimuth is saying, my experience and my limited understanding of the circuit is the opposite of yours @AdvanTech , its SS not Tubes that "crap the bed" faster/harder/strained in loud passages. Maybe I am not following you however...
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