1. There's no "rules" 2. Use your ears. 3. Go with your instinct. And use retailers that have fantastic customer service if you want to tweak, and I can almost hear quite a lot of you saying "of course you'll get good customer service from Snake Oil salesmen!"
I have a Denon pma 800ne amp, ifi Zen blue with upgraded power supply (but that's probably snake oil too eh? 😆 ) and Triangle Titus EZ speakers which all told cost about £1400 at full rrp. I have speaker cables that cost £130, interconnects£50 and a Tacima power strip that cost £36 so I am well above the 10% rule (even higher if you include the iPower power supply at £50). Did these make a difference to my system? Absolutely, I heard it with my own ears! Am I a fool? It's an irrelevant question, all of these tweaks I've made have brought me a sound I'm very happy with and I enjoyed the process of trying different speaker cables very much. Did I try slotting in a new component rather than trying different cables? Absolutely, a Node 3, waste of £550 in my system, sounded detailed but dead. Go figure...
Ooh, but where are the double blind test results and lab measurements?!
Yawn!