I like the idea of throwing a used mix-pre in front of the DR-100, but with the signal chain starting with the Oktava, I'm wondering what difference it makes. I've never been a fan of those microphones, even the "modified" ones. If your goal here is to learn the art of film making, then just work with what you have. Yeah, it won't be perfect, but that's ok. Nobody's first project ever is. I think your main goal should be figuring out workflow at this point, and learning how picture and sound go from production and through post.