Which is the most important part of a stereo system?
Please see https://systems.audiogon.com/systems/8367 and read the description wherein it says: "The System is built around the philosophy that everything matters. Everything either contributes or detracts, and no one single component is any more or less important than any other."
But hey, don't take my word for it. Check it out and see for yourself. Just try and play your system without a power cord. See how it sounds without power. Or speaker cables. Measure the sound output without speakers, without source. Pretty sure you will find they all measure the same: zero.
Which is what you will have if you follow the advice to put all your money into a source- or speakers, or any other component for that matter. They are all equally important. They all need to be good.
Which is not to say expensive. We all have our budgets. Within which everything should perform very nearly equally well. No weak links. If, and its a very important if, sound quality really is what you want.
We now return you to our regularly scheduled misinformation.