The power cord crack was a joke, but it wound up serving a purpose after all. More on that later.
As was said previously, the only "real" audible improvement will be by replacing your speakers, and $600 won't buy all that much of an improvement. anything else will be "incremental" and barely noticeable, and the cost will be out of line with the audible improvement. A more powerful receiver might help a little, but $600 won't really get you any better.
Do a search on the term "point of diminishing returns".
I suggest that, until you can put a finger on exactly what's got your panties in a bunch with your system, you simply continue to save and listen to speaker that cost considerably more than what you currently have.
Anyhow, it's now later. By your so easily buying into this "myth", which is a popular way to separate desperate newbies (and some not-so newbies) in this hobby from their money, you show that are susceptible to the
"puffery" that abounds in this hobby. Unlike Jack and the beanstalk, there are no magic beans in this hobby. Your best bet, for now at least, is to stick around here and read (and absorb) as much of the knowledge that's freely thrown around here before making any significant expenditures.
But, if you're bound and determined to throw good money after bad, have a ball. There are many other sites that will gladly provide you with enough ammo to drain your bank account.
P.S - click on the blue underlined word "puffery" in the body of the post. That blue indicates a link to another web site with more information on a subject.