HUGE gains are made once the install, source, and amplification are up to par. My amps always cost more than my speakers... further increasing the investment. Amps are much more complicated. And my install is worth >5x the value of anything (if you consider hourly rate and time).
this is what i ran before moving to Hertz Mille, they were around $700 new.
German Maestro Status SV6509 6.5" Set
as speaker quality increases, so does how natural the reproduction is. cheaper speakers add "coloration". the Mille are unlike any other speaker i've heard, incredibly natural and they lack midrange distortion common with many speakers (including high-energy).
but you need good amplification and a clean source to really let speakers shine. when powered by the stock headunit in my 2014 Accord, the Mille were awful... as would anything be.
there is a noticeable difference switching between analog and digital (fiber optic) as the source feeding my processor. my HU let's me do an A/B comparision with no delay between. with fiber optic, the soundstage increases in height, width, and improves overall. i suspect the improvement is partially due to better channel separation and cleaner high-frequency response.
when you want high quality at mid quality pricing, buy used. you can usually get speakers for 30%-50% of their retail price when buying used.