Poll: Most impactful component

A well known fact is that your preamp is the most important piece of equip. in your system.Upgrade to an Audio-GD tube pre and listen to how your whole system is upgarded.Speakers next.

If you have not yet experienced it, give BACCH a try.  It will improve any system, no matter the weakest link.  Pretty amazing.  

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.