We have two Marshall Acton Multiroom that have the exact same issue. Whenever they have been plugged in the power for more than 24 hours the sound becomes distorted/crackles and it gets increasingly worse the longer the speaker is powered on.
The only solution we have found is to pull the power plug for a minute before plugging it back in. Then the speaker will play fine for another 24 hours before it starts to crackle again. However, this solution is very inconvenient.
We have reached out to the Marshall support but they are not much of a help.
Has anyone else found a solution to this issue?