I found a couple of online calculators to estimate power requirements. From the Axiom blog, it seems 15 dB headroom is a good conservative amount of spare power.

https://www.crownaudio.com/en-US/tools/calculators#amp_power_required

https://support.biamp.com/Design_Tools/Amplifier_Power_Calculator

Both are problematic in that do you use the anechoic or the room sensitivity? There can be a 3dB difference between the numbers.

For me, 70 dB RMS at the listening position is uncomfortably loud, and my number is 2 W/channel to get my 15dB of headroom.

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