[FullRangeMan]

Many years ago I used the AQ Tweak Contact Enhancer on CD Player IC it had a surprising big positive effect on the music, but the audible benefit lasted only some hours.

[Zuman]

My personal experience is that I often won't notice any sound change due to a tweak when I install it. Sometimes, however, after having something in my system for weeks, months, or even years, I'll remove it. Frequently I'll then immediately notice a difference in sound, usually for the worse.
That's frustrating because it doesn't help me make improvements by "tasting" tweaks, but it does help validate their impact and encourages me to keep trying.
Difference-makers of note for me include: using a well-regarded switch for my Roon NUC/Server/Internet connection, as opposed to just plugging everything into the router; M101 Nova Ethernet cable; Taket Live supertweeters. I've had less success noticing a difference with isolation footers for both rack gear and speakers.

[rollo]

In the defense of tweaks... Plenty of utterly FREE tweaks around. The latest I tried was a gift from Michael Green who does 'tuning' as a professional. He suggested 'loosening the AC duplex, slightly" Holy Cow! it works! The sound became slightly less 'hard' a little softer, more natural.
Reminder FREE, Like FREE as in FREE. no cost. 
The free tweaks are the best. You can try it, or not, change it back, or not. no cost except a few minutes fooling around. Like speaker placement. 
Then more troubling.. footers.. There ARE dirt cheap good solutions, a buck each, even as low as fifty cents. Or you can splurge and buy $900 ones. EACH! ($900).
Most good tweaks started out as a free or nearly free IDEA. Grabbed by someone who managed to add in a profit. say 10,000%? Some folks take basic bits, claim they invented basic electronics, and then charge through the nose for it. (Bybee) when the fact is, anyone can make the SAME DEVICE for a few dollars in parts.
So IMO complaining tweaks are a 'rip off' is not fair. a lot (in fact nearly all) of them started out as an idea. a FREE or nearly free idea.

   A huge fan of Michael. Try taking the covers off. Cracking screws on enclosure, circuit boards, inputs and so on. Try woods under gear. The biggest affect came from removing plastic wire ties and separating the wires. Have fun trying.

charles