mudbuny sounds 'breaking' on sustained notes?

[add4]

I tested my mud bunny ram's head build with a bassist friend yesterday evening and when he plays loud sustained notes, the sound is not 'constant' but it 'breaks' from time to time .. makes me think of a badly biased transistor.. but it sounds great with my guitar.

we tested with 2 other muffs and a skreddy supa tone and it didn't happened..

I have no idea where that could come from... any suggestion?