The Denon X6400 ($2200 MSRP) is probably similar to the Yamaha RX-A3080 ($2200 MSRP) in terms of total power.
The RX-A3080 can output about 200 Watts x 2Ch into 4 ohms, so I think the X6400 should be able to power relatively efficient Klipsch speakers.
How far do you sit from the speakers? 15 FT?
My HT room is 22'W x 26'L x 14'H. I sit about 18 FT away.
Unless the volume is too loud, I don't think efficient speakers (90dB/2.83V/m or more) will need a lot of power.
But most people will buy amps anyway even if their speakers don't need the extra power.
I thought for sure I wouldn't need one but it seems like I have to get the volume up to get some decent sound levels...I'm about 20 feet away from the tv/front/center speakers