Hi Filipe,
Hum is a huge PITA, especially in a preamp which is compounded further by the fact most transformers are usually enclosed within the same box. I have overcome some input hum problems in the past by either careful routing and or using a metal enclosure which is also grounded to the star ground from the chassis( all inputs and output jacks being isolated), youll find more often than not you will have more hum problems with a wood enclosure. Failing the careful routing, or impossibility to do so, Ive always solved this issue with small gauge coaxial cable, this is well shielded and of very low capacitance for video/satellite reception use between the output of the pot and the preamp, this is the most critical point. The wire from the input to the volume pot doesn't matter too much, no need for shielding but wouldnt hurt to do so...
Hope this helps.
Colin