Generally speaking, vintage 2 channel amps should do well with lower impedance speakers as that is what they were asked of is to power 2 speakers. Like most things, the build quality of vintage units can be superior (though I don't know anything of this Onkyo). With today's electronics there is the tradeoff with features over power and build quality. With modern units it really boils down to features, purchasing power and (increasing) economies of scale of production.

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