Blown amp/receiver?

Greetings! First Post newbie.
I've got a Sony receiver model GX80ES that at least 15 years old and had it hooked up to some Bose Model 100 speakers. This was before I learned the ohms criticalness. The unit now craters when the volume is turned up about 1/3 the way (low end and high end). Before trashing unit I've seen some info that it may be able to be fixed. Any thoughts greatly appreciated! Regards, >dan