Sound recommendations?

After resurrecting an original Defender Cab ( http://youtu.be/WbpeIfumNkI ) I figured it was time to build a dedicated MAME cab. Component parts are an empty Beast Busters cab ( http://www.flickr.com/photos/33743995@N00/6439549215/ ) it was, literally, an empty cab. The art is perfect and staying untouched. Structurally it needed a little help as previous owners had...interesting ways of fixing things. I've got a Mac Mini for the brains, and a 19" LCD for the video.

The Cab still has the original speakers and I'm wondering if y'all have recommendations for getting the 'Headphone out' signal from the Mac, and smping it to drive the 4" (or so) speakers under the marquee.