What makes the biggest difference in sound quality?

Oldhvymec has it correct, electrical is the foundation and  without a proper foundation you will never hear what ur system is capable of .

Electrical
Room
Gear / vibration control
Bass / cabling / vibration control
Mains / cabling / vibration control

speakers and cartridge tend to make the most difference IME, but you reach a certain level where everything matters.  

I'm finding that the "level where everything matters" is as much a curse as a blessing. The Absolute Sound is like the Absolute Truth, doesn't exist, couldn't be found if it did, so the fun lies in the process of endless tweaking.

Drivers> Crossover>DAC>Source>Amplifiers>Cables>Room Treatment>Fuses/outlets.  

EVERYTHING matters. All of it, every part of the audio chain is critical.

Your thread isn’t working out quite as hoped aye Ted 😉