Professional Miniature Practice Bass Amp. Custom Design Passive Radiator The DOUBLE FOUR is the pinnacle of micro bass amps. With new driver design, acoustic loading and the latest digital amplifier technology, this tiny bass combo may be designed as a practice amp but continues to amaze everyone with the great PJB signature bass tone. From a cabinet with size of a jewel box, this seems to defy the laws of physics. Driver and Amplifier Technologies Driver and Amplifier Technologies Pushing our expertise of using smaller drivers to get the ultimate bass sound, we applied our vast experience to an even smaller, "Neo Power"4 inch driver built with precision of a Swiss watch. This 4" baby is able to cope with the extreme signals coming in from an amplified bass guitar. By using our extensive R