KEF LS50 Meta at 4 metres: how many watts does the room actually take?
85 dB average at the seat costs 16 W. A 100 dB peak at the same seat costs 506 W. Between those two numbers lies everything people mean by «is 45 watts enough».
For background and normal listening, yes. For peaks, no — and the arithmetic is unforgiving.
85 dB/W/m means one watt makes 85 dB at one metre. At 4 m you lose 20·log10(4) = 12 dB, so a comfortable 85 dB average at the listening seat costs 16 W. The Fosi V3's 45 W covers that with room left.
What sits above the average is the problem. A 100 dB peak — modest for orchestral or film material — needs 506 W at 4 m. The amplifier tops out at 89.5 dB continuous. Everything you ask for above that is clipping, and clipping is what actually kills tweeters.
What to do: move the chair, not the amplifier. Going from 4 m to 2 m returns 6 dB for free — the same 6 dB that four times the power would cost you. If the chair cannot move, this is the speaker asking for an amplifier stable into 4 Ω, where KEF itself calls for at least 80 W. Worth saying plainly: the LS50 Meta is a nearfield monitor being used as a room speaker, and its 85 dB sensitivity is the reason the room is expensive.