With the 4cm, one can set the REAL amp's power amp in the FX chain, thereby simply adding little touches here and there or just flooding the amp with distortion and effects...however you want it. If you turn OFF the poweramp setting in the FX chain and turn ON the GT8 preamp in the FX chain, you give the GT8 preamp a straight shot to your power amp, without any interference from the sound of the amp. In other words, you basically turn off your amp and the only thing that it's used for is a medium from the GT8 to the cab. In this mode, you can create your own distortion, effects, etc. I can create a 5150 amp with a certain cab, a Mesa Boogie with a certain cab, a Fender Bassman with a certain cab, or a Marshall Vintage amp with whatever cab you want. They are amp simulators, but the best that money can buy...and they sound like the real thing...almost. However, I choose to use my REAL amp's power amp in conjunction with some effects like a tubescreamer, Metal Zone, Overdrive, Big Muff, or whatever other stompbox you can think of because they're all built in. This is for distortion. Add wah, flanger, phaser, chorus, delay, or whatever you want, you've got it. There are limitless possibilities. I can use this setting and then switch to a new patch and it basically puts my REAL amp's power amp in standby and the GT8 gets a straight shot to it from the built in GT8 preamp. I can have 100 different amp tones all in one song all at the touch of a button...this is where the clean tone comes in. Instead of using a Marshall footswitch, the 4cm will bypass the need for it, so all I do is change to a different patch setting on the GT8 and BOOM...I've got clean sounds that you'd swear was directly from a JC-120. Tweak it a little bit with some chorus and maybe a touch of delay and you can do anything. However, in the 4cm, all the amplifier's controls are bypassed and you must control the volume and settings from the GT8. However, that's ONLY when the REAL amp's power amp is bypassed and using the GT8's preamp. I step on my second button on the GT8 for MY Marshall's sound boosted by a stompbox, compressor, EQ, and more all built in to the GT8. Now I need a clean break? I just tap the next patch button with my foot and since I have already created the settings and rearranged the FX chain, it will automatically turn off the REAL amp's power amp and use the preamp directly from the GT8, because I have a different FX chain in EACH patch.
The possibilities are limitless with this damn GT-8. It's amazing, except for the volume peda/wah pedal. It's got a really shallow throw. I'll just buy a Cry Baby and run my guitar through it and into the input of the GT-8 and I'm set.
I hope this makes it a little clearer.