LOL, exactly. You can have a ground loop if you have a ground wire connect to a volume pot and a tone pot that are also electrically connected via the control plate. In most humbucker's it is not a biggy but with single coils this is a source of hum that is not canceled but instead introduced into the signal of the pickup.
BTW: Hope the new preamp board fixes your problem.
In short, no. This is simply not an issue inside of a bass guitar.
There is only one ground (for all intents and purposes) inside of a bass. It is the same ground as that used by the amp being connected to, since the bass receives its ground through the shield of the cable. There is one (and only one) path to earth ground which happens through the cable and the amp (or any other device you connect to, like a DI, preamp, etc). Thus the ground loop problems that plague users of outboard gear (requiring isolations boxes, etc) just aren't an issue here.
I completely agree that the situation you describe can happen if someone were to solder wires from pot case to pot case: there would be multiple return paths to the local ground. I have no problem with that statement. However this is NOT "a source of hum that is not canceled but instead introduced into the signal of the pickup" and here is why:
In order for there to be amplification of a voltage, it must be in a position to modulate the active signal path in a way that will be picked up and amplified by whatever you're using to amplify the signal of the pickup. The situation described (having multiple paths for local ground) involve tiny fractions of an ohm difference in return paths to local ground. In addition, EMI coming in from the outside can only cause tiny changes in voltage due to Ohm's law (small fractions of an ohm with micro-amps of current = tiny voltage). There just isn't anything of consequence that can make it to the active signal to be picked up by the amp in this case. I would argue that induced EMI/noise through the bass's wiring (leading from the pickups to the output jack) accounts for only a tiny portion of the noise in the bass in general, which is why many manufacturers don't bother shielding control areas (aside from the cost reasons). Shielding the pickup cavities (where 99.9999% of the wire resides) would be the highest margin improvement, however for hobbyists you might as well do the whole thing, right?
If so, why do single coil pickups hum anyways? If you were to unwind a pickup, you'd find many feet of wire: in fact, in order to get 7k DC resistance from 40 gauge coated wire at 1ohm per foot, you'd need thousands of feet (7 thousand)! At these resistances, small noise currents end up being large voltages that can be amplified to audible levels. Viewed in this way, the foot or so of wiring leading from the pickup to the output jack makes up only a small percentage of the total wiring available to 'see' the noise. I bet this is why so many jazz bass users are disappointed after doing meticulous cavity shielding jobs.
I would hazard to guess that 99.9999% of the time people who shield their basses end up improving marginal ground connections which account for much of the improvements experienced.
In short, having multiple extremely low resistance paths to a local ground in the presence of tiny induced noise currents is not an issue inside of a bass. Having solid ground connections however is of paramount importance, however they end up being physically wired (through a pot case to a control plate, through a 'star ground', etc).
