The Marshall MR4100 JCM 900 is a 100 watt head that has been a favorite of rock and metal guitarists since the 1990s. It features two footswitchable channels, one low-gain and one high-gain, with shared four-band EQ and independent controls for volume, reverb, and gain. It also has extra effects including reverb, distortion, and looper. Here are some insights from user and expert review