Audio Systems: FET replacement in final driver stage, qsc power amps, curiosity sake

Question
A fellow student of mine at school was given a nonfuncional Sony 280GTX 480W car audio amp.  After checking the DC, we found a FET that showed a short from G to the DS channel.  Of course it needs replaced, but we are having a hard time finding schematics to tell wheather or not the device is part of a configuration that would require a matched set (i.e. class AB).  I'm fixing my guitar amplifier, which has a blown RC on the NPN side of the push pull, and I know that all the components on the driver stage need to be purchased from the manufacturer as a matched set to reduce crossover distortion.  My question is, does he need to get a part kit from the manufacturer.  Also, just for curiosity sake, why use FETs on the driver stage?  The only thing I can think of is that they are voltage controlled in the same manner as vaccum tubes, and for some reason Sony saw fit to controll the driver in such a manner, but dont FETs have a very high impedance looking in from the load (much greater than the RE' of a transistor)?  Wouldnt they need a coupling transformer to match the speaker impedance?

Answer
the mosfets really should be matched, but you don't need to go to the manufacturer if you can get them elsewhere, like dalbani electronics, tritronics, mouser, and the like. the main reason for the fet's is the switching time is alot faster, and there's less 'lead' and 'lag' at different frequencies. i'm a repair tech, not an engineer so i can't really discuss the different configurations some of the engineers come up with. just look at qsc power amps, all the collectors of the outputs are grounded.