Much appreciated if someone could tell me how good a MOSFET amplifier in a head unit is compared to some of the other unknown ones in other head units. i've noticed that all mosfet powered head units have 50W by 4 (that isn't RMS im sure) i don't want a seperate amplifier so just wondering how powerful these are?

The 50 x 4 quote is only peak power. Most advertising use a peak power figure as it is higher. A very quick and rough way of working out the RMS value is to halve the peak figure. The final figure is close enough most of the time. As for Mosfet, it is a Metal Oxide Silicon Field Effect Transistor, and are used in the output stages of the amplifier. The reason they have taken off in headunits, is that they have a faster response, sound better, and are more reliable. They also require less current to operate correctly, which reduces the space needed for the amp and can handled overvoltage/overcurrent.