Power rating does not change on tube amps!
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The maximum power capability of a tube amp will usually be somewhat different when a 6 ohm load is connected to its 8 ohm tap, compared to when an 8 ohm load is connected to its 8 ohm tap. How much different, and whether that difference results in more power capability or less power capability, and how much difference there is in distortion performance under the two conditions, depends on the particular amplifier.
The manufacturer could probably provide the best answer. Although if the amp also provides a 4 ohm tap I’d just try both taps and use whichever sounds best, without being concerned about the 6 ohm power rating.