I'm going into the CD input (RCA cable) on the back of the amp, then to my RP150. (shown in photo)
Most guitarists use 1/4" instrument cables, and go through the effects loop, or off the power amp.
The RCA cable method lets me bypass the preamp and speaker emulator, putting the signal between the preamp and the power amp.
You will notice a difference in tone on your patches, if you split your signal between the two blocks, (preamp & power amp) using the effects loop of the modeler,
giving you line level instead of instrument level. (less noise interference from your pups)
I prefer a tube amp with a few stomp boxes, and the perfect cabinet, for my own sound and tone.
You can't beat these processors for getting dozens of great sounds from a menu.
It's like a huge bag of Halloween candy, and you wanna try it all.
I got the RP150, because I like my Ibanez Screaming Demon way-wah, better than then ones on the modelers.
The GNX4 was discontinued, and too bad, it was Digitech's best by far.