Most useful tweaks that are sensible and really make a noticeable improvement

gillatgh ... with all due respect, your introduction to the topic doesn't fall under a tweak category, more under basic rules for achieving good sound.  Long time hobbyists would begin with great isolated electricity, then follow several of those suggestions as a lift off-off point.  Linn turntable's  Ivor Tiefenbrun, of course, asserted primacy "of the front end," a legitimate argue.  Once that is sorted out, and assuming the components have enough ability to expose sufficient resolving power, minutia and prat of a great system, tweaks can indeed prove fun, and quite beneficial towards one's goals and flavors.  If, however, you are in the camp that find wire upgrades, tube rolling, isolation and the like nonsensical, then, as with religion, no proof will suffice;  for believers, no proof necessary.  Free tweaks include leveling your gear - micro changes to loudspeakers, both vertically and horizontally (search info on bass nodes) - try live front wall, dampened real wall - address ceiling tuning if feasible - Marigo tuning dots skillfully applied to drivers, speaker baskets & cabinets of all sorts...see my responses elsewhere.  Experiment, be playful with it!  More peace, Pinthrift  

An obvious tweak is reversing interconnects to see which direction sounds best. In fact, there are a lot of things one can do to tweak interconnects, just to focus one one thing. Cryo them, freeze them, demagnetize them, apply anti-static agent or device, apply a smidgen of Cream Electret to the jackets, clean contacts, apply Graphene contact enhancer, elevate or suspend the interconnects to avoid vibration and static electric fields. Keep interconnects well away from power cords, at a minimum keep interconnects orthogonal to any power cord. There are even other ways to improve the sound of interconnects but alas, they are beyond the scope of this discussion, at least for the time being. Maybe later. 🤫

@geoffkait, I somehow fail to see the benefit of placing anything between points of contact such as graphene contact enhancer. In my simple mind a direct touch between contact points logically would provide the best contact. One would not place a graphene impregnated piece of material between contact points.
I do however agree with some of your other assertions. 

In reality graphene, with far greater conductivity than copper or silver, and by filling in microscopically small crevices, pits and pockets in what appears to the naked eye 👁 like perfectly smooth surfaces of the connectors, ensures much better physical and electrical contact and eliminates micro-arcing (noise) in one fell swoop.That’s why it’s a good idea to paint a thin layer of graphene on all (rpt all) electrical contacts in the house - audio and non-audio. Graphene also prevents oxidation and the distortion produced by dissimilar metal contact. As far as material that is impregnated with Graphene or multiple layers of Graphene goes the evidence seems to indicate they ACT like pure Graphene, or at least a very good facsimile.

Hey gillatgh - geoffkait

A very seasoned music lover and I paired up to try the Furutech Nano Liquid...I also shared some with another tech savvy friend out west...all three of us have had a very positive experience.  My west coast friend goes deep and wide with EXTREMELY subtle DSP...tiny 1/3rd db work...(hardly dad's slider eq -- for naysayers locked in to the 'less is more' mindset)...we work with Reaper and refined plug-ins.  He's a tough nut to crack and also endorses the Nano fluid.  I no longer use interconnects, just a digital cable.  The Nano was used for all power cables, speaker cables and the digital.  Cheers, Pinthrift