Your note on Linux being virus proof made me chuckle - a significant portion of worms and hacks are targetted at Linux systems.
They can run all the worms they want - but if the only thing running is NFS and SMB support, and the most trivial of web interfaces with no tricky server-side active content, then there's just not a lot to attack.

(rant mode on)
That's sort of the point. I don't want another computer. If I want to serve out music, all I should need is something that spits up files on demand. I could probably build it out of a PC-on-a-chip, DOS and a big disk, and it would be perfect - it would consume about 20W, it would be small and quiet, it would be virus proof and it would never, ever put me in the upgrade-or-perish position. But it's too much work - and devices like the SB demand you have something that runs their software in addition. Which means another computer. Twice the cost, five times the power consumption, a fan, and half the convenience.
I think of my print server as an example. It's about 3"x3"x2", with a network jack on one side and and a USB port and power adapter on the other. Power consumption is maybe a couple watts. Noise is zero. Yes, there's a computer in there, but I never have to think of it as one. I don't update it. It can't (AFAICT) get viruses. Occasionally it gets confused and I have to power cycle it, which really yanks my chain because I shouldn't have to fuss about printing when I want to print, but other than that it's perfect.
That's what a music server should be like. I don't want to log in as root, ever. Or do installs. At most, I want to turn it on and off, and that would be rare. My toaster works that way. Why doesn't this?