Thank you all for valuable advices.
I came across this info while browsing the Klipsch education pages: http://www.klipsch.com/Education/understand-receiver
"To give you the best balance of power and performance, select an amplifier or receiver that delivers continuous watts (many times erroneously referred to as “RMS”) in accordance with the speaker’s rating.
For example, a Klipsch RF-83 floorstanding speaker is rated at 250 watts. So, any amplifier that delivers 250w/channel would be a good match."
" The right amplifier, one that has its power well matched to the speakers, will also eliminate the need to overdrive components, which can induce clipping, which at best distorts the signal and at worst can cause serious damage. Believe it or not, underpowering speakers carries as much danger as too much power."
" Just be sure to keep the receiver’s output closely matched to speaker specifications to ensure years of trouble-free performance."
The speakers that I am planing to get are rated at 150 watts while the Onkyo tx-nr 646 delivers 100 watts at the same impedence of 8 ohms. The other one"ZR800" delivers 135 watts at 8 ohms and 180 at 6. Does this mean that I will be more safe with the expensive receiver even if the listening is the same?