Yeah, I think a PC in this instance is needless additional complication. I'm all for using a PC for media when appropriate, as I have an HTPC for cable recording and everything else the Roku can't do or access.
But for this, you just want internet radio services, I think a dedicated internet radio with integrated speakers is going to be the way to go.
I haven't researched Grace's quality just yet, but I'd have my eye on this one:
http://www.amazon.com/Grace-Digital-...AEDKAFERZNM3VB
Stereo speakers for one thing. And I think the others are just as capable of it, but you can also use a USB drive and play FLAC files if you so choose. If the device is capable of producing the extra clarity, who knows.
Integrated devices like that can be a bit scary, and so the add-on internet device might not be a bad idea in the long run. If you want to push some serious tunes in the garage, you can get whatever boombox or stereo equipment you desire, and add that second item I linked.
If you simply want some basic tunes to pass the time while you're working on things in the garage, but won't be hosting any parties or whatever in there, one of those integrated devices will probably be for the best, simply due to the ease of use.
I MIGHT look at Logitech devices simply for the peace of mind of warranty support - I've constantly heard of easy support and device replacement, sometimes even out of official warranty IIRC. It looks like the better model from Logitech only has a single speaker (the one with Ultimate Ears branding), but they also offer a more basic stereo box, but it is not color and it may not be as capable as the UE model or the Grace models.