Probably blew your tweeters. If you like to listen real loud then almost certainly that's what it is. Way to test, find any other speakers you can and see if they make that same sound.
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You verified that the problem is the Yamaha. That receiver is about 40 years old . I would let it go and buy another receiver instead of spending money to have it repaired. That’s up to you!!
This new Yamaha has all the features you need!
I suspect that the bias diodes used to give thermal feedback from the heatsinks are shorted. This will cause the amp to sound exactly as described. It causes the amp to have extreme crossover distortion. I would take it back to the service shop and have them finish the job. They should have tested the amp prior to sending it out- they should have caught this.