That "dirty up the signal" bit is spot on, my old Echoplex adds this grit to everything that no plugin has ever gotten right for me. Made the switch two years ago and my board has never sounded better.

Reading up on this last year when mine did the same thing, the tube icing is usually a sign the water valve is letting a little water seep through when it should shut off all the way. That slow drip hits the cold tube and freezes, over and over, until it's a solid plug. Mike's heat gun trick only clears the symptom, and every thaw cycle makes that plastic weaker, which is why yours cracked on round two. The real fix is replacing the valve and checking the fill tube alignment so water doesn't sit in there, or just living with it and defrosting the tube with the unit unplugged. Some folks also put a little foam wrap on the tube, but the valve is the root of it most of the time.