My Hypex amplifiers gets very warm, is this normal?

Class D amplifiers are highly efficient in terms of power dissipation. If a Class D amplifier were heat-sinked like an equivalent Class AB amplifier, it would remain virtually stone cold. The primary objective is to reduce the heatsink size, making the amplifiers more compact. An idling UcD™ amplifier consumes up to 2% of its rated power—equivalent to 8W for a UcD400 amplifier. Whether this results in a significant temperature rise depends on the size of the heatsink. For further details, please refer to the "Thermal Design" application note.