What a rude response. I was trying to help you! I ain't stupid but your description of what you are trying to do is rather vague. If you are so dammed clever I wonder why you cannot simply research suitable solutions online, beyond 'Googling for pedantic threads' without asking dumb questions on a forum....
Anyway - perhaps you should look at Audio over Ethernet solutions - sometimes called 'stereo over Cat 5' you can purchase units that work in pairs over CAT5 cable from around £30 - I'm not sure how well they will work within a network that includes Ethernet switches or whatever (they, or some, should do as the better ones perform well in various industrial/professional situations including in stadiums) - if you still want to be networking the PC upstairs to the 'hub' downstairs over a single cable you will need to either add switches or another dedicated Cat 5 (or higher) cable for the audio feed.
Use phono cables to connect your PC sound card at one end and amplifier input at the other. The main drawback with this can be the lack of control.
Or maybe you are looking for some other solution that might include a 'share' of content on the PC with another device local to the amp.