Thanks for the help guys. I ended up ordering a cubic foot of resin from affordablewater.us for $110 including shipping. They list it as "Dow American Made 8% Crosslinked Cation Softening Resin", but they don't say exactly which Dow resin it is. That's okay, because even if they did say which one it is, I wouldn't know the difference between the others if there even is one. Also, I have a Fleck 7600 not the 5600. After the resin arrives, I probably won't feel like rebuilding the valve and I'll just call it good.