Actually, the Fender will play the tracks without a guitar plugged in. I actually don't need another amplifier. I just want to alter or augment the one I already have. The sound is actually very nice; however, after you play a tube amp you can feel the difference when you switch to a transistor and I sort of miss that feel.
The preamp sounds good.
Speaking of no tone controls, I always disconnect the tone controls on all of my guitars. I want as little interference between the amp and the guitar. Any extra tone either comes from my fingers or I get the amp or a pedal to create it.
The thing about the Fender G-DEC is being able to play with an entire band at a button. I love that.