If the amplifier sounds OK running it at half volume - as in not obviously distorting it should be fine.
The various 'power ratings' seen on such equipment don't mean much, but be aware even a 25 Watt amplifier can blow up '200 Watt' speakers if it is overdriven into distortion.
You can test the system with an oscilloscope on the amplifier outputs to determine the clipping point, then always keep the output level lower. You should/will hear the point at which amplifier distortion begins with your ears so the previous is not really necessary.
An amplifier of such age is probably full of dust and fluff which should be blown out to allow the heatsinking to work properly. Take the lid off and blow the dust out carefully, gently, with compressed air if it is obviously dusty. It's not a good idea to brush the dust out and use a vacuum cleaner as such things as preset pots for DC offset and bias can be disturbed.
The old ROTELs are reliable amplifiers and much better than any modern shit of comparable recent prices - manufacturers could not afford to make and sell a similar product these days due to the much higher raw material costs of copper, steel and aluminium. To illustrate this point just check the weight of an eqivalent modern unit compared to yours !
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