MOTM

A little help through your precious opinions:
In the manuals of all MF models it is indicated that the units can be 
powered up to 15V so I often use 12V instead of 9V i.e. well the only 
problem is with the MF104, at 9V it works fine, if I select 12V the 
dry signal is still fine but the wet is distorted and a consistent 
hum (rather loud) is added to the sound. When it first happened I 
wrote to moogmusic, I was using only one type of wallwort at that 
time with voltage selectable and 1A of current, they said it is 
strange, it seems that F104 doesn't like 1A and 12V, now I tried with 
all kinds of power supplies and I can say that the problem is still 
there at 12V, yet I can say that at 9V there's no problem but I can 
not say that the sound of the unit is so fantastic so maybe there's 
something wrong and defects are justs enhanced at 12V? Any thoughts?
Many thanks
Enrico (Italy)