Which 4 Ohm speakers?
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Take your pick, many modern speakers are 4 ohm impedance.
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What is an 8 Ohm amp?.
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Take your pick, the thread is vintage, and only a few of the vintage amps would reliably drive a low impedance load.
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So by your way of reasoning, an amp rated to deliver 80, 100, 150W into 8 Ohms would be "sketchy" delivering 50W into a 4 Ohm load? Alright.
^yeah exactly^
My reasoning is that any amp, which looses its magic smoke when pushing a 4 ohm load, is not a 4 ohm “capable” amp.
If the amp does not say “4 ohm” somewhere on the box, label or in the manual, then I would be a bit hesitant to hook it up to a 4 ohm or 2 ohm load.
But I am not overly brave, so YMMV.