Fortunately, thanks to @Droopy_TX, any overwritten stock presets are easily reset using the preset templates linked in the Droopy_TX Preset Experience sub-section of the Tone App Support section of the Rumble Club Wiki page. He painstakingly captured all the settings in all the stock presets of the Rumble Studio 40 & Stage 800. Dunno about LT25 presets, though.
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You can do a factory reset from the app or directly from the amp with a combination of button presses.

