Why Tube Amplifiers - Comment from Ayon Audio site

After decades of solid state amplifier innovations, tube amplifiers have staged a powerful comeback in the last ten years, due to their sonic merits.
Simpler, purer circuits
At Ayon Audio, we feel that tubes are the sonically superior technology for audio, as their generally simpler circuits and smaller number of components provide for a purer signal path and consequently more truthful signal handling. This is because fewer components provide for fewer elements in a circuit to degrade the signal, distort and muddle the sound. Simple circuits also provide for inherently higher reliability, since there are fewer parts which can fail. Tubes are also more tolerant of circuit drifts and deviations in component specifications, and thus can be used in simpler, purer circuits.
More benign overload and distortion behaviour
A lot of music features great dynamic signal swings, and it has been well established that in tube amplifiers the onset of clip/overload as maximum power is reached is gradual and rising distortion is of predominately low even-order harmonic nature. In comprehensive listening tests, even high levels of even-order harmonic distortion has been found to be significantly less offensive to the ear than even small levels of the harsh, odd order harmonic distortion produced by solid-state circuits when their reach their power limit and enter clipping. In transistor amplifiers, the distortion rise very quickly as the maximum power level is reached, showing almost square wave characteristics, and a high DC component, - which can destroy easily loudspeaker drivers if not stopped to do so.
Vacuum tubes and “Tone”
The difference in the distortion characteristics between the two technologies can be well illustrated by looking at their effects in guitar amplifier design.
Tube guitar amplifier manufacturers have traditionally designed their circuits to drive the output stages into overload distortion, using the resultant distortion to achieve their trademark “tone”. In a tube amplifier, this tone contributes to the amplifier's sound, but in a solid-state amplifier this distortion is audibly intolerable and easily destroys the speakers. 
When transistors overload (in a discrete circuit or in an OP amp), the dominant distortion product is the third harmonic. The third harmonic "produces a sound many musicians refer to as blanketed”. Instead of making the tone fuller, a strong third actually makes the tone thin and hard. On the other hand, with tubes (particularly triodes) the dominant distortion product is the second harmonic: “Musically the second is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller”. Tubes sound better because their distortion products are more musical. Tubes provide a more appropriate load to transducers. Those are the fundamental reasons why tubes simply sound better.
Vacuum tubes are the more linear and require less feedback
Tubes are voltage amplifiers as opposed to transistors which are current amplification devices. As a consequence, tubes are a more linear amplification technology, requiring less overall negative feedback to make the circuit linear. Negative feedback re-injects a sample of the amplifier’s output signal back into the input, 180 degrees out of phase, in an attempt to reduce amplifier non-linearity and distortion. In practice, negative feedback tends to slow the amplifier down and sucks the emotion and life out of the music. High feedback designs usually sound sterile, boring and lifeless, while low or zero feedback designs provide for a more immediate and natural sound. Depending on technology and type of the used output device, transistor amplifiers generally require the use of over 40dB of local loop or global negative feedback.
Superior dynamic capabilities
The higher working voltages present in tube amplifiers generally allow for wider voltage swings and better signal headroom before entering into overload territory. Higher working voltages yield higher audible energy storage* with lower value capacitors. ~ 500 volts working voltage in a tube amplifier approximate about ~ 80 volts in a transistor circuit. This is most likely why many listeners feel that tubes sound more powerful