It depends. Many bookshelf speakers are not sensitive but also don't have deep bass response. When SPLs go up to high levels (and power draw goes high), usually it is at the lowest frequencies (below 40hz). You will rarely see frequencies above 60hz rise more than 10dB above average volume levels even in the most dynamic music.
I have speakers here with a frequency response that begins rolling off at ~60hz and 45w is plenty to drive them at reference levels despite having an SPL of 84dB. If they started rolling off at 40hz, I would be far more concerned and would seek a lot more power.
I have speakers here with a frequency response that begins rolling off at ~60hz and 45w is plenty to drive them at reference levels despite having an SPL of 84dB. If they started rolling off at 40hz, I would be far more concerned and would seek a lot more power.