Most often the output MOSFET's and associated circuitry will burn out when the amp draws excessive current. And excessive current draw is most often caused by operating the amp into an impedance that is too low. Of course, depending on the age and actual use, the amp could have failed from normal use. The US Acoustics USX-4085 is a relatively good quality low-end amp that was manufactured with excellent components.
All of their amps manufactured after 2001 carried a 3-year warranty. But when operated in "tri-mode", with normal speakers and a subwoofer, the subwoofer minimum impedance rating was 8 ohms. And the normal speakers should be operated through a high pass capacitor while the sub needed a 15.9 mH low pass filter inductor.
The MOSFET's and resistors can probably be replaced at a qualified electronics shop. If you are somewhat handy with a soldering iron and have a controlled temperature soldering station, you can do it yourself. Parts are available from: http://www.allelectronics.com/
Hope this helps.