I had some spare time this morning and I gently removed the pots to have a look at them. They indeed had the a and b marking and guess what... I accidentally used the a pots for tone and the b pots for volume... So I have a job to do!

Unfortunately it will be next weekend because the whole familiy is coming our way for dinner and that means lots of preparations in the kitchen and diningroom

Anyway, I'm glad I know what the cause is!