No so much "standard" as most common.
Just to be clear, there's nothing wrong with 6 ohm for the Denon 1613. But you had said you were picking the Yamaha's *because* they were 6 ohm. That's what I was objecting to.
Also, 映画ファン has a good point, other than actual sound preference...
It doesn't make any sense to buy speakers based on their ohm spec unless you have a receiver that won't play that low. In this case the Klipsch's are 8 ohms, and are therefore MORE flexible for use with other receivers. It's best to audition and purchase based on sound preference, my next...
As long as the receiver/amp can handle *less* than what the speakers are rated at, everything is good. So the receiver can handle 6 ohms, yours are 8 - you're good. Even when the "equation" comes out bad there's some give, as long as you don't crank the amp. The worst case is when the...