Double Four 70W Bass Combo Amp Black. The Double Four is a single channel bass amplifier that has an auxiliary input for iPad/iPod, mp3 or drum machine. It features 3-band EQ that is perfectly tailored for bass guitar, headphone out jack and a line out for recording. It is ideal for all kinds of basses and can handle the open B string on a 5 string bass with ease. The cabinet may be as small as a shoe box but it is built to the same standards of all PJB cabinets, heavily braced internally and damped with acoustic damping materials to prevent cabinet coloration that can kill the true sound of your instrument. What you hear from the Double Four is the closest you can get to how your instrument actually sounds. The Double Four may just be a practice amp but it does not mean that it has to sound like a practice amp. The sound is huge for its size and you may find it hard to believe that all the sound from your bass is just coming out of such a tiny jewel box.