Not nearly enough information provided - so I'll lead with a couple of caveats.

All in one boxes like that are typically designed to get the most from the least. Meaning, that sized box and amp were chosen, because that's what the woofer could handle. To keep budgets down, not much, if any headroom is left over; the sub was probably at it's physical limits, and the amp at it's electrical limits.

Typically I feel what helps a sub is #1 - a better amp, one that can handle more current, has better power supply to provide that current. #2 more power, not for volume, but for that extra headroom - typically  an amp that's hardly working sounds better than one that's chuffing away at 98%

I would just get a decent plate amp and get the sub working again. If he wants better, he should look into a better sub, or better yet, another sub to even out the frequency response in the room.