The problem with some wireless units has to do with the little transmitter units that plug directly into the output of the bass. Extraneous noises occurs with some of them because of the close proximity of the transmitter to the active preamps (NOT the pickups, which aren't active at all unless they are certain EMG pups). It's impossible to know which combination of bass and plug-in transmitter works, but I would avoid those units. There's several reasonable-priced units where the transmitter has a short 1/4" cable. This is enough distance between the transmitter and the bass's electronics to noise getting back and forth
This situation concerns me because of where the noise is coming from and going to. If it were because of noise getting into the preamp from the transmitter, wouldn't it cause a feedback loop that could cause damage to amplifiers and speakers, not to mention the PA the bass is connected to as well as the ears that are in the line of fire? Or if it's noise from the bass's preamp getting into the transmitter, that speaks to the quality of the transmitter unit and is cause for concern, too.
The only other problem an active bass could cause was mentioned earlier and could be a very hot signal from the bass overloading the transmitter, which can be easily solved by dialing down the sensitivity of the transmitter pack (if it has such a control, I've seen a couple that have no such control), dial down the output trim pot of the bass's preamp (again, if it's so equipped, which drove me nuts about the smoking-hot, but otherwise great preamp in the Fender American Elite basses - no output trim control), or simply turn down the volume of the bass (which would also drive me nuts - hard to exactly duplicate the level each time it's played).
Shure, Audio-Technica, and Sennheiser packs are equipped with sensitive controls, as is my Lectrosonic belt pack (for $1400, it had better....). I don't know about the Line Six systems as I've never used one or looked at them.
Hope this rambling is of some help.
This makes a lot of sense to me. When a transmitter is plugged right into the bass and is right near the active preamp, without effective shielding that RF can creep right into the preamp where it can cause many problems. It can be like picking up a radio station to just changing the bias on circuits. All of this can cause changes in tone much the way a case of parasitic oscillations can. The deal is that the RF signal strength of the wireless falls off as the square of the distance from the antenna so putting a little distance between the transmitter and the active preamp can cause big improvements in this kind of problem. The reason actives are sensitive is that often the inputs of preamp ICs can act like radio detectors injecting the problem right into the bass circuits. Passive circuits do not have this problem (unless you have put diodes in there for some reason) so that with a passive bass the RF has to travel all the way back to the amp to cause problems. The amp is probably some distance away.
