As the latest portable speaker system-slash-case from Portable Sound Laboratories, iMainGo XP ($119) is the company's first iPad-specific product, and is substantially different from its prior iPod- and iPhone-compatible speaker cases. iMainGo XP now packs four speaker drivers rather than two, and uses a fully hard plastic chassis with hinges that flip the speakers open to the left and right of the iPad's screen when viewed in landscape mode. A rechargeable…