Patrick,
PlanetPress does do it's own conversion to eps in the background, using it's own resolution settings, such as those in the Document's "Resource options". You can try to increase the values there. For best quality, use 2400 for the resolutions, 100% compression level, or change Photo to Line Art. Then reimport the pdf files(if they are not dynamic), and make sure that the Picture objects that displays them is the exact same size as your page to make sure there's no downscaling.
Let us know if that makes a difference! (Note: using top quality settings might make the job run slower. High quality = less speed, and there's not much we can do about this.)
Regards,
Raphaël Lalonde Lefebvre