Ashdown ABM 500 Evo II sub-harmonic weird behaviour

Hi everybody,
I have this strange - to me - thing going on on the sub-harmonic feature of my ABM 500 combo: it kicks in only with relatively high gain (i have to keep it at 2 o'clock at least if I play normally). If the gain is lower than that, to hear the effect I have to play louder and it will last only for the few moments after the attack.

I wonder if this is a normal feature or there's something going wrong there.

I have to say that other than this the amp works perfectly and I am very satisfied with tone and power.

Anybody can help me?

 

...