Try this:
Add a local user on the computer that you're trying to shut down (the one that has the mp3's).
You can do this using the Computer Management control panel (Control Panel\All Control Panel Items\Administrative Tools\Computer Management) in the Local Users and Groups section.
Name the user the same as the user on the computer that you are trying to shut down
from (the user on your sofa PC).
Use the same password (if any), and make sure this new user is a member of the administrators group.
Give it a reboot for good measure and then try that shutdown command again.
This may seem strange, but in the past Windows OS file sharing would bypass login prompts and such if you set up users in this manner, and I wonder if this might work for the shutdown command.
If you are looking for a NAS device, I would recommend a
QNAP.
Personally I have the
QNAP TS-509 with 5TB.
I have it set up so I can do a WOL through the internet (like when I want to use it from work), and it has a web interface that you can use to shut it down.
I also have it set up to automatically start and shut down at certain times and on certain days of the week (I have it set for it to only be on when I'm at home and not asleep).
It's a Linux box so you can mod it to run other programs. For example, I put a different torrent client on mine.
It doesn't use a lot of power, when it's active I think it uses about 60W, and when it's idle it's less than that.