DenZeten:
This should normally be a very simple matter to sort out. There is no reason why the wah pedal should sound or behave any different when using the G30 in front of the wah instead of a cable before the wah. Except from the "no-loss" treble thing. Most guitarists worldwide (incl. me) are only using certain types of pedals/effects (in a spesific order) in front of the amp while other effects again are hooked up in a spesific order to the amps effectloop. Although there are no written laws/rules when it comes to how pedals/effects should be organized, I have seen over the years (I have played guitar for 31 years and I am also a classically trained pianist) that most guitarists have pretty much the same understanding of how these should be set up. I am not an American but from what I understand here your set up is very simple: Guitar - G30 - wah - amp (input front) and then the effectboard are hooked up to the effectloop. If so, this is the correct way and how it should be used.
Keep in mind that wah pedals in special (due to their internal components) are much more sensitive and likely to produce unwanted noise/hum if they are placed near power adapters who again creates a "magnetic field" around them. Although the magnetic field is quite "weak" and doesn't effect other pedals again to the same extent. That's why many guitarists incl. me are using the popular American Voodoo Lab pedal power 2+ which gives 8 separate 100 % isolated 9v DC outputs which eliminates ground loops and noise that could occur if normal single adapters had been used for each pedal instead. If that wasn't enough, It also has a built in toroidal transformer that eliminates the magnetic field and the final result is up to 8 dead silent pedals/effects.
In your case, my personal guess is that you are experiencing a ground loop. Just for testing, given that your setup is like described above and your wah is working normally when using a cable and if the wah is powered by an adapter, I would suggest that you try a battery instead and see if that solves the situation. If it does, you should then check how your adapter is powered. Your entire setup should be powered from the same "source" and not several different 120v circuits. You should also try to simply "relocate" the wah from the rest of your gear and see what happens. If possible, try another wah if you have access to one to see if the same situation occurs. Your wah could be extra sensitive to the magnetic field from power cables and the adapter that goes with the wah could also be of poor quality. Other obvious things is to make sure that you are using high quality shielded cables (not longer than needed) and keep all power cables and adapters separated and away from these.
Again, this is very easy to fix and one of the steps above should normally do the trick.
Best regards,
Tiger
