FX Loop +4db or -10db settings:
Audio equipment has these two as "standards" for how big a signal can be +4db = Line and -10db = Instrument level.
If you have a switch you set it to whatever the equipment on the other end of the cable is using, if both have switches then go for the higher level on both.
But as with most such "standards" there is a wide variation in what specific gear can output or receive and they often don't match. The only thing you can do is experiment with the settings until you find one where it doesn't overload when you want clean sounds. The HD500 has Line and Instrument settings that match the +4 and -10 settings on the JVM so both are adjustable.
Series FX Loops interrupt the main signal so that the FX Return is all that goes to the power amp, Parallel FX loops pass the primary signal though and you want the FX return signal to contain only effects without the original signal. For using the HD500 you want to be using Series which is 100$ wet on the amp.
This link might help you: http://jvmforum.com/....php?f=9&t=5333