I don't like the sound of a PF(Diatonic)–>Distortion fx chain config. The harmony notes sound smeared and muddy. If your amp has an fx loop, you could try the PF there so it's placed after the preamp. Unfortunately, then other effects that come before it in the chain like delay won't sound good with a PF Diatonic patch. If you also need delay or modulation on the harmonized riff, I think the best bet is to switch your amp to a clean setting when using the PF and instead use distortion from a stombox placed in front of the PF.
