Poll: Most impactful component

As many others have said speakers are probably the part that can change the sound the most. Still, an unbalanced system can sound worse than a slightly cheaper but well-balanced system. And changing just one component in a balanced system to another that is much better and maybe much more expensive will make the system unbalanced. 

For example, a system with $100k speakers and a $1k amp might sound a lot worse than a system with $10k speakers and $10k amp.

If you have a well-balanced system and suddenly get a lot of extra budget to improve it it is probably best to replace many parts at once.

Fuse connecting turntable, anyone?

I would opine that the preamp is the brain of the system.

The synergy of all the gear together and the resultant sound is the soul.

@therandyman 

So the BACCH is a fancy room correction ? What exactly does it do for a price tag of over 50k? 

I bought BACCH4Mac, which cost about $7500, including a new Mac Mini computer with 16 GB Ram.  That includes a camera for head tracking.  A system without the head tracking is much less expensive.  It eliminates crosstalk between your ears.  The effect is that on well-recorded material, the soundstage is hugely widened and deepened.  For example, on a 1958 Sonny Stitt record of a jazz quartet, in my listening room the sax is in the kitchen and the drums are outside the house!  This is without adding any distortion.