I don't own Synology NAS, but I have read some about it a while ago and understand what capabilities it has. IIRC you can install media server plugin to it and have it stream media files to upnp or dlna capable devices. I had a brief look at Synology's knowledge center on how to do that and I believe you need something in the middle if you need to connect to fiio k7 or k9. In simple term, you need a media renderer to output to your amp. A mobile phone would work or something like Raspberry Pi /SBC if you are a DIY type.