I know it's probably a Thermistor or even a Thermal Fuse it's located in between the MOSFET's but the sign they have has been used for NTC or a Varistor as well in SMPS could I use a NTC or Varistor and if it's burnt how will I ever find one of equivalent values

 

If it is a class AB power amplifier, and if the device is attached to the same heatsink as the power transistors, and if it connects to the circuit somewhere near the output stage then it is probably a thermistor for temperature compensating the bias current.
If it doesn't connect near the output stage, then it is probably for an overtemperature shutdown.