4. An amp modeler is an a piece of equipment which simulates other amps by trying to give you the closest sound to the amp you want. Something like the PX4 does this.
Thats all i can really answer so far. I'm not too sure about anything else, sorry.
By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.
