I'd like to mention that there are different kinds of "noise" produced by bass circuits/pickups, and that each has a different cause and cure.
First, there's higher pitched humming caused by a badl ground- this can be as extreme as an instrument with no ground or the "normal" kind with stock instruments, where as soon as you take your fingers off the strings/other metal parts you hear a high pitched hum. With a good shielding job, the circuit doesn't need your body to provide a good ground (for techies reading this, yes, it's more complicated than this, but I'm typing on my lunch break so bear with me). So after a bass is properly shielded with a star ground, there will be no high pitched hum even if you aren't touching metal (in fact you can remove the bridge ground completely).
Then there is induced noise, from lights, dimmers, CRT monitors, TVs, and transformers. This can be a combination of hum and hiss. This is picked up along the path from the pickups to the output jack. Shielding eliminates this, but this noise can be present even if you're using humbuckers or other noise-cancelling systems. This noise is introduced after the pickups.
Next, there's 60 cycle hum, which is a low hum (on guitars you hear the octave above at 120Hz since guitar amps don't do much at 60Hz, but with a bass amp you hear both). This is just part of single coil design, and you can't eliminate it with shielding, though shielding does reduce it a bit. As above in the thread, you can reduce it further by shielding the inside of the pickup covers, but there is a tonal tradefoff. John Suhr has an upcoming system that will eliminate this, but as of this writing it's only available in his factory guitars, not aftermarket or for bass, but in the future this may be an option.
Last, there's what I called "filtered" 60 cycle hum, which is often present in humbuckers where both coils aren't exactly equal (few are). In this case, some of the hum is eliminated but some isn't, and the phase of the hum is altered, causing cancellations. This results in a lower level and higher pitched hum, but it's still 60Hz hum. Just like single coil hum, this is impossible to truly eliminate but you can minimize it. But at least this is low enough in level that in a well shielded instrument it's not a problem.
Not all of the above applies to active pickups, though it all applies to active preamp circuits. Every instrument can benefit from shielding.
Last, I was recently raked through the coals on another forum for recommending copper foil. The guy said in effect "no professional luthier uses copper."
Aside from that being total BS, I'd like to talk about copper vs conductive paint.
Both work. The paint is easier to work with, though slower (takes many coats with long drying times). The copper is harder to work with, though faster to apply. The copper gives a better shield, but the paint gives a very good shield. Paint is more expensive in small quantities, but if you're doing a lot of guitars, paint is the way to go. Copper is removeable, paint isn't (not at all easily, at least).
So Sadowsky uses paint in his production instruments. But if a client wanted him to shield a 50s Strat, I'd bet Roger would use copper since there would be no permanent change to the instrument.
I use copper as I only shield an instrument when I buy a new one (typically one every few years, though I've been buying a bit more than that lately). Each time I get a new instrument, I order $20 worth of copper shielding and spend an hour shielding the instrument. If I were doing all my instruments at once, I might spend $60 for the paint and give the whole project a few days to allow for drying time.
So there's the whole copper vs paint dilemma. Beyond that, some people just like to argue.
