I can typically hear a noticeable improvement at sometime around 1 AM to 2 AM every night ( or is it morning ??? ). Since i live on a relatively quiet block next to a field, i don't think it is the ambient reduction in noise level that i'm noticing since it is relatively quiet most of the time. My guess is it is the difference in AC quality, as the music sounds cleaner, sharper and more liquid with a blacker background. This is quite noticeable even on a quality FM station such as WFMT.
As to the functionality of some tweaks, they are undeniable. When your cd player / TT are hop, skipping and jumping due to heavy footfalls or acoustic feedback, you CAN correct this via "tweaks". The results, when done properly, are undeniable. No double blind tests required or scientific studies needed.
As to some of the other more esoteric stuff, i would not have believed some myself until i noticed it first hand.
Being an electronics tech for a living, i used to argue tooth and nail about "power cords". An AA inmate ( Ozzy ) and a few other guys went round and round with me and a few others about this subject one day several years ago on AudioReview. I don't think that there was ever such a heated nor fast paced exchange on any subject there. There were over 60 posts in a matter of appr 4 hours.
Needless to say, i was talking out of the top of my hat i.e. i had never used an aftermarket power cord since i considered it "foolish". Well, once i had tried a highly touted power cord ( Custom Power ), i was a believer. While i did not like the specific effects of that cord on the specific component that i tried it on, hearing the difference DID make a believer out of me. I now own quite a few aftermarket power cords and also build my own.
Changing racks also had a MAJOR effect. So much so that the tonal balance and spacial characteristics were altered. Believe it or not, i had to change amps to get back to where i was before that rack. Who would have thought that what the component rests on could affect the sonics of the system ??? While i could understand a TT being affected by such things as micro-vibrations, etc., who would have imagined a pre or power amp sounding different ??? Needless to say, i am now a "believer" in the mystical art of tweaking.
Stereophile did an article about "proper placement" of equipment on racks, where to place your footers, etc... several years ago. This article DID have some scientific reasoning behind nodes / standing waves and energy transfer between components and shelving. Moncrief also covered some material like this in IAR, but concentrated more on the mating surface / points of contact between the component and the type of footer being used.
The thing that makes all of these points "controversial" is the different levels of hearing acuity & listening skills. If we all had the same hearing ability and listening skills, the playing field in terms of "tweaks" would be a lot more level. Since this is not the case, we simply have to rely on what we think we hear and experience. What sounds good / bad / different to you may not be the same as what someone else hears / experiences. Sean