Digitally stored sound is not 'less warm' than the same sound in analog storage, unless it's degraded by bad compression. The way valves (and analog players) 'warm' the sound is by adding harmonic distortion. Not exactly 'hi-fi' in my book...
As for streaming, most devices has limited file support. You'll have to find the right match between streaming device and source (computer/software or NAS), to ensure both support the formats you want to use. With a bad match you might end of with MP3 only...
If you want wireless streaming, you must also check how the streaming device will work with your existing network. Most devices only support 802.11g, which means you'll need a router that support multiple 'network speeds', or reduce the speed of your data transfer. Devices that use their own network might still interfere with your data network (with the risk of glitches in the streaming), and anyway they will be closed systems that don't allow you to stream audio or video from your computer.
As long as you use lossless files and keep away from systems that compress the sound (that is BlueTooth), the sound quality will depend on the digital to analog converter. Very few streaming devices will have a DAC that can compete with a separate one of reasonable quality. With a modest stereo system the built-in DAC will do though.
If you want to use a computer as your main sound source, Apple's AirPort Express and/or Apple TV will be an inexpensive and very versatile solution. With a separate quality DAC and a quality stereo system the sound will be as good as with any other system.