Of course, closed-minded strictly by-the-book meter-reader engineer types who believe that the differences obvious to audiophiles between cables, amplifiers, use of "tweak" accessories, etc. are hype and illusion will not be interested in the following, and need read no further, rather than make snarky comments. I'm an electronics engineer myself, and I have found through long experience that these effects are definitively real and pervasive in audio, and at least partially can be corrected by various measures.
This is a complicated topic. I agree that well-conducted objective measurements, up to a point, are totally valid for what they can show. The issue is, there is a vast amount of subtle audio information important to audiophiles that is real and physical but at the same time is essentially not measureable because of the limitations of normally available instrumentation equipment. But this information is still perceivable by audiophiles and is very important, because the resolving power of the experienced audiophile's ear-brain system is greatly beyond the capabilities of electronic and acoustic measuring instruments.
Caveat: for high quality/high cost components, and "tweaks", to be of benefit your system already has to be good enough in terms of resolution, imaging, etc. for these forms of system modification to be of benefit.
Just one area of "tweaking" and the effects of high quality and expensive components on the sound: the area of cables. Two of these subtle mostly unmeasurable in practice but extremely important effects are the improvements gained by high quality cables via two interrelated mechanisms: increasing the time coherence of the system, and also by reducing the noise floor. To "improve time coherence" means to reduce the delay and smearing of sonic energy of a musical event over some period of time following the event. This "time smearing" phenomenon is inherent in the mechanical and electrical systems used for sound reproduction, and the ear-brain system is very sensitive to it. Electrical examples are skin effect or frequency-dependent phase shifting of signal current propagated through the interior of the wires in a cable, dielectric absorbtion in cables and capacitors, and microvibration-induced timing distortion. With digital cables this time smear phenomenon translates into noxious audible distortion because it causes timing jitter in the digital data transmission. In analog cables this phenomenon directly translates into audible noxious distortion.
Mechanical / electrical examples in other areas of high end audio are the time delayed and resonant behavior of speaker drivers and enclosures, flutter (rapid speed variations) in turntables and CD transports, and time smear induced in the phono cartridge output due to stylus contact-generated energy returned to the stylus after first being propagated into the tonearm and record. Interestingly, timing jitter in the CD playback serial digital data is caused both electrically and mechanically by vibration, and rapid speed variations in the transport drive mechanism.
Another example of vibration feedback-induced time smearing is the vibration of wires in cables due to sound pressures from the speakers and to electromotive forces induced by adjacent current-carrying wires.
Yet another form of mechanical vibration-induced time smearing is the fore-and-aft vibration of a speaker enclosure in response to forces on the driver voice coil. This is simply due to Newton’s law of action and reaction and occurs regardless of the rigidity and degree of damping of the enclosure. Simply placing a 15-20 pound lead weight on the top of the speaker improves clarity of sound considerably by reducing Doppler distortion due to the reactive fore-and-aft motion of the enclosure. Doppler distortion smears sonic energy over a range of frequencies (rather than time) and is inherent in all speaker designs. If a driver diaphragm is moving at both a low and a high frequency at the same time (say 50 and 5000 Hz), the higher frequency is modulated (distorted) by the lower frequency due to the Doppler effect. As the sound source approaches at some velocity its sound is shifted up in frequency proportionately to the speed of approach, and vice versa for the sound source moving away from the listener. This effect "frequency smears" the output of all speakers, with the effect worsening with decreasing efficiency, smaller radiating area and 2-way designs. Of course the weights also improve performance by increasing the damping of cabinet resonance.
The common effect of all these and many other time and frequency smearing mechanisms is a massive perceived blurring, smearing, flattening and veiling of the sonic "picture", along with various tonal imbalances such as overbrightness and bass boominess or looseness.
Tweaks of various types and more expensive and more sophisticated components partially correct these fundamental problems such as time smear and RF induced noise.
This analysis is partial and evolving and only touches on a vast subject. One of the fascinations of audio is the complexity of the basic problem — attempting to reproduce recordings as realistically as possible in a home environment. The elements of the problem include electronics, psychoacoustics, acoustical engineering, mechanical engineering, physics (electromagnetics) and many other disciplines.
