Plastic probably isn't your issue. As long as you're using food grade plastic which is what most homebrew shops sell.
Phenolics can come from a few different sources such as fermentation, grains, water source, infection, chlorine etc. What kind of phenolic flavors are you getting? Spicy? Clove? Band-aid? Tannic? That could help narrow it down. My guess, since the beer is an APA and you aren't getting any hop character, is infection. And if you don't notice it prior to kegging, I'm assuming it's happening somewhere in the keg. I would always take apart the posts and dip tubes to clean, sanitize and reassemble between each batch. There are quite a few good cleaners and sanitizers out there. I use PBW to clean and Star San to sanitize. Hope that helps.