Not a laptop, it's just a LIVA X I got from local store for about 50 euros.
The machine is headless running an modified core Windows 7 image only with the essentials and a micro WiFi card on one of the USB ports and a large flash drive on another port.
The phone is RDP-iyng into the PC.
Running Stereo Tool 7.82 with Busybox HD preset with no Declipper and no Natural Dynamics, at 50% CPU Quality. Seems to handle this very well (~60-85% CPU load with Winamp, Crossfade plugin, Soundwire and AWARemote server). One caveat is that it has about a second or so delay and upon changing tracks it has some kind of quick skipping of a second or so ahead and then the next song plays. Wasn't able to fix this by adjusting the Plugin section of the Stereo tool.
I'm sure this can be optimized further but this will do well for now. It's small enough to hide well under the car, it consumes about 3.4 Amps at 5 Volts from the car audio's 12V dedicated outlet.
More info, videoclip demo & more to come
The shown above is just a teaser.