@HenrikTJ, truly appreciate you taking time to compare and report the test results of your both speakers! It's definitely helpful, but made my decision bit more complicated. :-) Your suggestion about the AVR (Denon 3400H) tweaking is very good. Will do some more experiments with it too.
#Ceptorman and #willland, I called up Klipsch Cust Support (always on long hold, for 40 min this time around, but get the great service in the end) this morning. The Tech absolutely recommended RP-404C over either RP-500C or RP-600C. He said that the outer two subwoofers handle < 500hz frequencies and the inner 2 subwoofers handle between 500hz and 1500hz. The twitter handles anything above 1500hz, so rp-404C is a 3-way speaker in comparison and it will sound better than those 2-way center channels.
@#Ceptorman, I asked the tech about why RP-8000F web page on Klipsch site recommends RP-404C as the center channel, and not bigger RP-504C. He said that it must be an error on the web page. I also note that not all Klipsch site web pages are updated (e.g. they still have "Tweeter & Woofer Removal" PDFs for old RP line of speakers, and not for newer line of speakers).
My thoughts at this point: Since the upgrade cost is affordable, the form factor of RP-404C is smaller (height) which will fit better with the larger size TV upgrade in the future and hopefully it will sound equal, if not better, to my ears in my room, I am considering to move forward with the upgrade.
Thanks.