The CHALLENGER series of monoblocs are the foundation of the AudioValve range of amplifiers, which have been refined and perfected since first introduced in 1987. The CHALLENGER is unique in so many key areas. For example, it can use KT-88s, 6550s or EL-34s with equal ease. And depending on the tube used, its output can range from 110 wpc to as high as 150 wpc. This is made possible by our revolutionary and proprietary ABR circuitry, which monitors, biases and controls each tube independently.