some of the best sound quality cd players are sony. hard to say which one for $100 with line out and mp3 support though.
two i know a bit over $100. the sony d-ne900 has a pseudo line out. its headphone jack is also a lineuot and optical out. you can change it from headphone out to line out within the menu. there is the sony d-ne920 which is similar to 900 but is $150. then there is a slimmer, alluminum one i think called dne10 which is pretty similar.
and the sony d-ne1, my personal fav. it has as seperate line out. i get 35hours with mp3. it never skip on me.
both discontinued, so you have to hunt for used one. the dne900 are made after the dne1 and it has parametric EQ. the sound qualities would be a worthwhile upgrade from iriver and you get longer batery life too.
the top of the line sony is dne20, that one should be excelent but costly.
panasonic have some good ones like sl-ct710 and sl-ct800, 810. but no line out and cant fast foward with mp3. it seem the sl-ct720 have that pseudo line out kinda thing as well, and probably the sl-ct820.
i'm not that impressed with iriver since the battery life is lower comparing to the competion. sound isnt flat. and they skip on me. the sound might be good if you use the optical out with the imp550 though, but i know no cheap portable amp that actaully use a digital connection.