[ofcmetehbset]

I'm an owner of a Sennheiser HD598, a Xonar DG soundcard and a Yamaha RX-V463 receiver.
 
My wish is to listen to music through the receiver, but the manual says the headphone jack has 470Ω, while the phones are 50Ω...
People said that low impedance headphones plugged into a high impedance jack could damage the phones.
 
But I'm not sure when this happens, I mean, do my phones get damaged the moment I plug them in the jack?
Or after having played music on them for a long time? Or do they only get damaged by extremely high volume?
Or is all of it just nonsense?
 
I really want to use the receiver to play music on instead of my soundcard, but I'm fearing the headphones will degrade rapidly with improper source.

 

[NA Blur]

I think the biggest concern is more about hearing noise.  With high output impedance you are probably going to hear some noise in the system as well as have poor control of the drivers on the headphones.  See damping for more details about that.
 
If you want to protect against possible damage try these steps:
 
1.)  Turn off the receiver
2.)  Plug in headphones
3.)  Turn on receive
4.)  Gradually turn up volume until music can be heard at normal to low listening levels
5.)  Listen for any severe distortion.  Turn the receiver down and off if you hear any severe distortion with your headphones
6.)  If you do not hear any loud pops or really bad distortion you are going to be just fine.

 

[goodvibes]

It won't damage them unless you play it too loud. The headphone output circuit is likely just 470 ohm resistors inline from the amp.