There are SPECS...which are based on Bench Tests using Dummy Loads with Exactly 4, 6 and/or 8-ohm PURE RESISTORS....and then there is REALITY.
An Amplifier is a CONSTANT VOLTAGE source....which means that when a Signal Generator at a fixed Voltage Level (RMS, Average) performs a Frequency Sweep on the Input, then the VOLTAGE OUTPUT on an Output Channel (or Channels) will also be a constant Voltage Level, although at a different Voltage [Po = Vo^2 * Z].
However, when the same Frequency Sweep is applied to a Dynamic Speaker (i.e. S-W or Woofer), then the amount of CURRENT sucked out of the Amp depends on the Impedance at each Frequency. The Highest Current (and hence most Power = I^2 * Z) is drawn in the 300-500 Hz region for most small Woofers [look for or MEASURE the Voice Coil DC Resistance = Re in the Specs] and perhaps 60-250 Hz region for Sub-Woofers, although it varies quite a bit with S-W Drivers varying from 8-in to over 18-in Diameter. Speaker Impedance is actually MUCH HIGHER in the vicinity of the -3 dB Roll-Off Freq and hence draws considerably LESS Current (and Power) to reproduce the Lowest Freqs. Also note that Amp Tests typically use a sustained sinewave signal....so we don't usually learn that the PEAK Power the Amp provides [which is what we SHOULD be worried about to avoid Clipping] is typically 10 dB HIGHER than the "Spec" number:
[Typical 12-in Woofer]
Fortunately in most Music, Peak Power is also "typically" 10 dB higher than Average....so we can justify talking about AVERAGE SPL provided by a given Amp AVERAGE Power Rating...and perhaps IGNORE that pesky 3 dB AMPLIFIER HEADROOM "Fudge Factor" in fol. Room SPL Calculator:
Also bear in mind that Mid-Range and Tweeter Speakers are only lightly driven compared to Woofers & Sub-Woofers....AND frequently have an L-Pad Attenuator to reduce their SPL vs the Woofer's more efficient SPL. So only a SMALL FRACTION of the Musical Spectrum actually requires significant Current to be sucked out of the Amp....and it does NOT include those very LOUD, sustained Low Frequencies....but WILL include Drum Transients and also even louder CRASH, BANG, BOOM!!! special effects....where you would be hard pressed to KNOW whether your Amp went into Clipping or NOT.
If you could attach a DVM to your speakers output while playing LOUD Music....or even better a Real Time Spectrum Analyzer running on a PC, such as FREE TrueRTA or FREE REW (Room Equalization Wizard), then you would experience an AH-HA MOMENT....the AMP is usually NOT providing a very high AVERAGE Voltage....and the PEAKS are NO WHERE NEAR the Max PEAK Voltage that the Amp can supply. Your EARS will usually Overload long before your Amp.
Fol. series of posts provides specific info re. how LOUD your ears can tolerate before they no longer provide any musical enjoyment, depending on how long they are exposed to excessive Sound Levels:
Note that Sub-Woofers are a special case, since human ears can tolerate MUCH higher SPL Levels (Euro Max Specs are 130 dBC to 145 dBC, depending on country).]