Play music from laptop over WiFi to existing audio system: how?

Save money. Use existing smart phones or tablets (don't buy custom streaming devices) and Install plex server on laptop (free). and create a library. Install plex app on a separate phone laptop or tablet (~$5 each). Connect digital out to dac and dac to pre-amp. Or use analog y connector from headphone jack to hifi/preamp input to use dac in phone or tablet. Cast from plex on laptop to device connected to hifi. Play music and enjoy. If you have another phone or tablet install plex app there too and use that as a remote controller to the device running plex that is connected to hifi.

Is Plex server available for Linux?

> Connect digital out to dac and dac to pre-amp. 
Digital out of which device is to be connected to DAC? You are saying don't buy any device, so I don't know which device you are saying will connect to the DAC. Or are you suggesting installing Plex on a dedicated computer (like a NUC) and positioning that near the DAC?

@jondThanks for the mention of AURALiC. I am not familiar with them or Bluesound. It does look like a couple AURALiC products have the features I was seeking:
AURALiC ARIES G2 Wireless Streaming Transporter
AURALiC Aries Wireless Streaming Bridge

In particular, I would like the device to have a USB output that can be connected to the USB input of my DAC. However, since I am already happy with my DAC and all I am seeking is to replace the USB cable with WiFi, I don't understand what value the AURALiC products add that makes them worth several thousand dollars. There are $50 Bluetooth music receivers that support the aptX low latency protocol. Something like that would almost be sufficient, it seems, except they only offer RCA outputs and I would prefer WiFi instead of Bluetooth.

It seems that there should be a range of products in between these low end Bluetooth music receivers and the AURALiC Wireless Streaming products. The AURALiC products cost more than my DAC and I'm happy with my DAC. Apparently I do not understand the value-add of these products.

Check their web site. I believe it is available on linux.

You have to run plex server to host a library. Plex app or plex.tv in browser accesses Plex. With any home computer you can run both on same linux host and do usb digital or analog out from there. Or you can run plex app on any supported device and connect that to the server.

@mapman thanks for the tips about Plex. I see that the Plex server is available for Linux. Plex does look interesting and I will look into it further; but I cannot see how it solves my present problem. I still need a wireless connection to my DAC. The problem I have is where you said:

Connect digital out to dac

I am presently doing that with a 30 foot long USB cable and I wish to make the connection wireless. With Plex, I still have to do that -- or I could set up a small Windows NUC at my DAC and run the Plex player on it, but that's not the kind of elegant solution I'm looking for. It adds yet one more computing device that I have to maintain into my system. I like my present system and I mainly wish to replace a 30 foot long USB cable with a wireless connection.