If you have the patience to manage Windows - and it can be frustrating..
get a custom PC - fanless, SSD with a linear PSU. You can get a lower power processor and use one of those fanless casings which double up as a heatsink.
Run Windows Server 2012 and JRiver. You can tweak it further - use JPlay and the Windows Server optimiser.
If you don't have the patience to manage Windows, get a Mac Mini from one of those companies that specialise in modifying them and adding a linear PSU. As before, use an SSD
You can JRiver for Mac, or iTunes with Audirvana Plus
get a custom PC - fanless, SSD with a linear PSU. You can get a lower power processor and use one of those fanless casings which double up as a heatsink.
Run Windows Server 2012 and JRiver. You can tweak it further - use JPlay and the Windows Server optimiser.
If you don't have the patience to manage Windows, get a Mac Mini from one of those companies that specialise in modifying them and adding a linear PSU. As before, use an SSD
You can JRiver for Mac, or iTunes with Audirvana Plus