[Mr Bill]

as no amount of replenisher will remove the silver that is already in the fixer, unless you remove the solution itself—now, if this is incorrect, please let me know, as I am eager to know.

Well, although this is correct, it's not the whole story. When you replenish, you don't REMOVE silver, but the fresh replenisher being added will dilute byproducts; thus you control silver concentration by changing the replenishment rate (solution being removed changes volume, but not concentration).

I don't know that there would be any advantage to replenishment other than the tanks staying in a steady-state condition. You're already doing good by using a 2-bath fixer and John's Silver Magnet. (Anybody doing any volume without something like the Magnet, or better, is missing out in my opinion.)