The amplifier inside the receiver normally generates heat, especially when run at high volume for sustained periods.
This is why receivers have a metal heat sink or vents on the back, to allow the heat to dissipate from inside the receiver.

• While some heat is normal, the unit should never be too hot to touch.
• To minimize this heat, it is a good idea to:
  1. Place your receiver above the other components in the rack or cabinet.
  2. Provide several inches above and behind the chassis of the receiver to allow heat to escape.
  3. Use an Audio/Video cabinet with an open back for ventilation.
• Some receivers have a speaker impedance selector switch on the rear panel. If your speakers are 4 or 6 ohms, set this switch for "less than 8 ohms".
• Low impedance speakers cause the receiver to work harder, and this will cause high levels of heat as well.