I assume these are single coils? Are the pole pieces shielded, i.e. copper screened and grounded to earth on the jack socket, pot or other ground point?
[b]My interpretation of your problem:[/b]
From your description, by turning the treble all the way up you are magnifying the sound. Alternatively, by turning the bass up you are diminishing the sound... although the same interference will be present.
By touching the strings, bridge, pole pieces etc you are soaking some of the interference (sound you can hear) caused by the magnetic field of the pickups. Your bridge will be earthed (assuming this has been done correctly)
I may be wrong though A little more information may help.
Stu