At typical average listening levels, speaker is probably demanding between 1-5 watts of RMS power (depending on the sensitivity of speakers). So even 10 watt amplifier seem to get as loud as 100 watt amplifier.

What set these two amplifiers apart is not how loud can they get, but rather how can they deliver momentary musics "peak" passages (that can reach up to 200 watt) at low distortion levels (i.e, THD%, S/N). So the more [reserve*] power your amplifier have, then it is better equipped to handle music power bursts.

*Reserve power is sometimes refer to as Dynamic Headroom of amplifier. So an amplifier with DH of 3 dB can [momentary] deliver twice its continuos [RMS] power.