I'll keep an eye out for Soundstream's stuff.
I used to (well, still have) an empeg. Originally a british company, the unit came with one or two laptop hard drives, ran a customized linux kernel on an ARM processor. MSRP was around $1200 at the time, they just couldn't compete. SonicBlue (now Rio) bought them, decided to EOL it, liquidated their stock of empegs, and I bought one for $500. Absolutely gorgeous machine: the unit was fully removable to move between vehicles, play music in your house, had interfaces to sync your music on your computer, do play lists, all kinds of awesome things, and a great interface to boot. Very easy to navigate your music, very straight forward, but it was a single purpose device: to play music from a hard drive. That's it. But that's all I wanted.
Unfortunately, they were EOL'd quite some time ago, close to 10 years at this point. I should've bought two, as mine's no longer working.
So I've been trying to find a worthy replacement, and candidates are lacking.
Everything I've been able to find has already been talked about in this thread: Sure, connect your iP[hone|od] to pretty much any modern receiver and it'll work - if you have one of said devices, and are willing to deal with the otherwise crap interface for managing play lists, rewinding, go to next song, etc. Okay, so pretty much every modern receiver also has a USB port where you can connect an external hard drive - but everyone I've seen is limited to a single partition of FAT32 with all the limitations listed in this thread. Oh, and if you want to specify an order of play, you have to name your files in alphabetical order, and creating directories as your play lists and navigating through them is tedious at best. Not a great option. It works, sure, but if I were to deal with something as archaic as that, I'd get an iPod - if they made one that would hold my entire music collection.
Why it's so hard to read m3u files? I couldn't tell you. If they did, I might actually give the USB option a try.
There are some manufacturer options (Lexus?) where it comes with a hard drive that you can put your music on, and seems to work reasonable well (though I haven't actually seen one.) But I have yet to find a useable aftermarket unit that'll do something .. useable.
I'm even going to begin the search for threads on how to build you own linux based car stereo, because the sheer lack of options is astounding for something I consider an obvious next step in car stereo evolution.