The Panasonic SL-SW505 does, in fact, have a line-out. It is located on the back of the unit, next to the DC connector. Both of these are covered by the rubber gaskets that make this unit (and other Shockwave models) water resistant.
Your 505 is very similiar to my 405, which has been in use for several years now, shows no signs of wear at all, and is still my favorite for portable use. I have used it directly from of its headphone jack, and from its line-out, with several cans and amps. I find both outputs to be of good quality and sound.