I know nothing about the specific internals of this speaker's electronics. But … here's what I suspect is the most likely scenario, and worth exactly the 2 cents I charge
Unlike a typical analog amplifier system, the gain control is likely not just a pot in the audio circuit. It is controlling the bias on some other solid state or IC stage in the Class D amplifier, which ultimately controls the overall gain (volume).
Since the volume level is "stuck" at max, the odds are that there is some solid state device or chip somewhere that's died and gone to heaven … not just the gain control attached to the knob.
If you had a schematic, it might be possible to puzzle out the offending device and replace it. If not .. maybe Adam would sell you the electronics package at a cost less than buying a new speaker (?) And if they will but you or someone you know don't feel confident to attempt the replacement, or if the cost of shipping it to Adam for repair out of warranty is more than the cost of the new one, then …put it in the chipper (and maybe save the speakers and other mechanical parts for a future project).
On the other hand, maybe you just need a new gain control. What you DO need is a schematic. I told you this was only worth 2 cents