Many thanks to all for your quick and good answers.

I already knew Mark's web site http://www.mhennessy.f9.co.uk/mf_a1/

Unfortunately I cannot replace the preamp: I need some cash so I am
thinking to sell the A1, sigh sob :-(

I am looking for the potentiometer, but here in Italy it is very hard
to find.

I write to Musical Fidelity. If they don't reply, I will order at
Farnell, even if shipping cost will kill me!

Best regards, Massimo