Thanks for pointing out my typo, yes it is indeed $500 difference ($5K is quite insane

). At this moment I only have 5.2.4, I noticed 4500 can only have Height 1/2 to be assignable, while 6500 can have surround back to be assignable too, 4500 limits the possibility of bi-amp if I will go for rear Atmos speaker. Again I'm quite puzzled if bi-amp will make my RP-280FA better or not (My current amp doesn't have bi-amp). What do you think about the rear Atmos speaker? Does it really give you a better experience?
Thanks for your comments. Bi-wiring should not really change a lot compare to bi-amp, just curious how much of the difference with bi-amp/non bi-amp? When you said the crossover setting at THX recommended frequency 80Hz, when the receiver is at Direct mode, I thought this crossover will be bypass, is that correct? The reason I choose Denon because of its well known Audyssey, it seems like most people get the best measurements from Denon's receiver, just don't know if this is true to believe. Also 6500 is made in Japan, I don't know if this contribute better QC of the receiver, or I should not even worry about it.
If I have another stereo amp for FL/FR, should I connect it with single connection/bi-amp?
Currently I'm doing my little research and prepare for Black Friday sale, but I will think about the external amp option for my FL/FR.