Actually, you are half right, from their FAQ:
This is a decision made by each individual AVR manufacturer and each uses a different speaker roll-off frequency to make this decision.
That is to say, if a speaker is found to roll-off below 40 Hz it should be called Large and all other speakers should be called Small
So how do you know what your AVR manufacturer uses for a speaker roll-off and what is the frequency roll-off for the M80's, M60's? I have an older Rotel processor, the instruction manual just says set your mains to large if they are, well large. I would bet most people would view the M80's to be a large speaker, yes, no?
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