The Two-Rock TRCU50HDSS Custom Reverb v3 SS 50W Tube Guitar Amp Head is based on the same platform as the previous versions, but added are some new features to increase the amp's flexibility. This includes a full active loop with a "Return Level" and "Pull Bright" function. Also, you'll enjoy the dual contours and the ability to use with 6L6's or EL34's. The "Pull Mid Defeat" will allow you to scoop the mids, giving the amp a more "fender" style front end. The SS in the name refers to the rectifier that is solid state in this model.