Amplifier power output does not affect sound. An amplifier with 10 watts of output power will sound the same as a 100 watt amplifier. The difference is loudness. At higher volume levels to double loudness it takes 10 times the power. To double the loudness of 10 watts it takes an amplifier power of 100 watts. To double loudness of 100 watts you need 1,000 watts of amplifier power. Amplifier power is not the only factor that affects loudness. Distance from a speaker and efficiency of the speaker are also factors. In an average home room it is the speaker efficiency that is the most critical. The higher the efficiency rating of a speaker the less amplifier power is required for a specific loudness. Rather than using amplifier output watts as a gauge for loudness use sound pressure levels (SPL).
When considering sound pressure levels think about sounds in the environment. The following is a comparison of everyday sound pressure levels in close proximity based on average published values.
dB — SOURCE
40 — Quiet Library
50 — Average Home
60 — Speech
70 — Vacuum Cleaner
80 — Side of Busy Street
90 — Diesel Truck
100 — Jack Hammer
110 — Chainsaw
120 — Pneumatic Drill
140 — Train Horn
Sound pressure levels above 85dB are high enough to cause some hearing loss depending on exposure time. The higher the SPL the more chance of hearing loss. At an SPL of 100dB you really should be wearing ear protection. At 120dB without ear protection the chance of hearing damage is high even for short periods of exposure. At 140dB ear damage will most certainly happen without ear protection. If you would wear ear protection out in the environment, then why would you want such high SPL in your listening room?... unless of course you plan to wear ear protection.
Assembling an audio system using 85dB as reference for maximum SPL would provide substantial loudness. If paired with the right speakers, 12 amplifier watts can provide an SPL of 85dB. Sensitivity of speakers is usually specified as so many dB @ 1W/1m (1 watt at 1 meter from speaker). For example, 93dB @ 1W/1m. Higher sensitivity speakers are more efficient and require less amplifier watts for a specific sound pressure level. A pair of speakers with a sensitivity of 96dB @ 1W/1m requires less amplifier power than speakers with a sensitivity of 93dB @ 1W/1m. With speakers having a sensitivity of 93dB @ 1W/1m 12 amplifier watts will provide an SPL of 85dB 20 feet from the speakers.
At crownaudio.com you can use the 'Amplifier Power Required' calculator to determine amplifier power output. Listener distance from speakers and speaker sensitivity are the prime factors. Enter the listener distance from source (speaker) in meters. If 20 feet, then enter 6.1 meters. Enter the desired SPL level in dB at listener distance, for example 85. Enter sensitivity of the speakers, for example, 93 (93dB @ 1W/1m). Amplifier headroom at 3dB to accommodate audio peaks. Entering those parameters you should find that it only requires 12 watts of amplifier power.
For music a normal SPL listening level is around 70dB, 80dB is loud. Sound pressure levels above 85dB can cause hearing loss especially with long continuous exposure. A home audio system designed to produce an SPL of 80dB to 85dB ten feet (or twenty feet in larger rooms) from the speakers should be sufficiently loud. If you pay a little extra for more efficient speakers and drive them with a lower wattage amplifier, over time you will save money. Besides the savings in amplifier costs lower power amplifiers draw less current with lower electric power costs. Vacuum tubes used in lower power amplifiers are less expensive, a savings each time you replace tubes.