This is tiny, inexpensive and easy to hook up- unbalanced in and out though, Used to be available at Maplin.
http://www.kemo-electronic.de/en/Light-Sound/Amplifier-Splitter/Modules/M040N-Universal-preamplifier.php
Also, the Radio Design Labs STM-3 data sheet says it will operate at 12V with reduced headroom.
http://www.rdlnet.com/downloads/datasheets/stm-3.pdf
Cheers,
D