Many good points here gents!
I do not agree that all amplifiers sound "the same" at one Watt. However, most amps will operate in Class A at low output, and definitely sound their best there - it may well be that they sound less different at one Watt.
I would add that the laws of physics define a relationship between low bass limit, efficiency, and box volume. So for a bass woofer, the designer who wishes to increase efficiency must either compromise bandwidth (low frequency limit) or make the box bigger.
Cheers,
CRP