Establishing permanence and repeatability is a mechanism to control for, or at least provide statistical significance in the face bias and random outcomes.
No. Permanence and repeatability don't control for bias.
If you do for example a proper level matched double blind ABX test then you have controlled for bias, and by repeating the test often enough you get statistical significance.
(In case you don't know ABX: the test subject can listen to A, B, and X as many times and in whatever order and as long as he/she wants and then has to indicate whether X is the same as A or X the same as B.)
Are you suggesting that the bias is permanent and the suggested effects are not? Please say more.
First let me say I use the word bias here in a broad sense, let's say meaning any form of perceptual error caused by something other than the actual sound. I don't know if this is completely correct usage of the word, but it saves me a lot of typing.
There are many forms of bias, and bias can work in many ways.
Just an example what could have happened in the case of the usb drives etc.:
Maybe the first time that you listened to the "bad" usb hdd for whatever coincidental reason you perceived the sound as you described ("flat"?). At that moment your subconscious could have concluded (or at that moment you concsiously concluded) that the usb hdd must be the cause. At that moment a permanent (or semi-permanent, for an undetermined duration?) bias could have been created in your subconscious mind that causes the same "flat" effect in your perception in later listening sessions.
Examples of possible "whatever coincidental reason":
-In the case of headphones: a (slightly) different positioning of the headphones on your head
-In the case of loudspeakers: a (slightly) different position of your head in the room (even a few inches can make a difference: for higher frequencies with short wave lengths there will be a fine-scale interference pattern of dips and peaks, different for different frequencies, accross the room, in fact it could even be that at later listening sessions you subconsciously move your head to the same or a similar sounding spot in an effort to confirm the "flatness" to yourself - an idea suggested by Ethan Winer if I remember correctly).
In the beginning I read all your posts but I have stopped doing that, all the possible guesses of what might cause a real audible difference are tiresome. The chance that bias / perceptual error is the cause is astronomically larger than the chance that there is a real audible difference. I hope this post helps you to understand that. Bias isn't rare, or easily predictable, or confined to one form or another.