This is quoted from the vmps website.
"None of the VMPS Subwoofers are powered (i.e. have built-in amplifiers) for a number of reasons. Installing an amplifier inside a woofer enclosure, the common practice with "powered" subwoofers, invariably leads to amplifier failure due to vibration literally shaking the circuitry to death within a relatively short period. Examine the warranties on powered woofers and discover that many expire within six months of purchase, or warrant amplifier components for a year or less. Worse, the quality of such built-in amplifiers, particularly the dreadful Class B and Class D designs now popular, is poor; often the entire amplifier component is purchased offshore from a vendor for around $60.00. Whena built-in crossover is provided, it too is generally of minimal design and execution, with the cheapest parts and wiring. Such built-in amps are often far from linear, cannot pass even a good sine wave at any frequency, and are burdened with contouring (bass boost of various sorts), infrasonic filters which ring and color the entire woofer output, limiters to prevent the user from playing the sub louder than about 100dB SPL at 1m, and other exigencies which severely limit dynamic range, output levels, and distortion. VMPS Subwoofers require no equalization or filters, since all three extend response well into the
first octave at very low THD and high output levels. Any main system amplifier of good quality and 20W rms per channel or more may be used, and the Subwoofers will uncomplainingly accept prodigious amounts of power."
