Tube amp tone it the stuff of legend. Since the inception of the electric guitar, musicians have sought ways to emulate the sound and response of a full-up, hard working tube amp without the hassles and high volume levels. Sure, tube amps can add grit to your tone, but there is also an x-factor - the tube "sag" that gives that biting compression and singing sustain that is nearly impossible to achieve without having your axe in close vicinity to a cranked amp.
SD-2 SUSTAINER D page at Guyatone 's website...