Please someone help!

I presume you got it second hand. If new... take it back to the store. Neither series nor parallel should hum. My Warwick Corvette $$ has a series/singlecoil/parallel switch. Only the single coil is susceptible to hum, like a Jazz Bass would. You should retain humbucking ability both in series and parallel, by design.
If second hand... I'd check the wiring myself. If it seems complicated, I'd remove everything and wire it from scratch, carefully, making sure you shield the cavity and all grounds are ok. It's easier than troubleshooting.

 

There's always some hum/buzz out of the G&L MFD pickups when running them in series mode. My L2000 gives a pretty hefty buzz in series mode and my L2500 does it too tho not quite as bad.

The MFD pickups are extremely hot with loads and loads of output so they just pick up a lot of hum. In parallel mode they calm down a fair bit and seem to have some hum cancelling.

If it's a really loud buzz that overpowers the output of the pickup, then you probably have a problem.

Otherwise, I'd just use a noise gate when using series mode. It is kind of a drag that they do this....

LS

 

Mine is dead quiet in either mode.

 

Treble boost usually means noise boost as well.

 

Mine never hums either... I would even maybe try cleaning out the electronics or taking it to a tech... I believe if you look in the G&L forum though there might be a fix in the stickied thread.

 

Full treble boost can make even a pretty quiet bass noisy.