Hi wizard. I have done each and my preference by far was the Metal Magic alone version for the reasons you cite
In my case, I was specifically treating for iron with a vinyl liner and lower ch, with a sand filter. I'm assuming you're treating for iron. (If you're treating for copper, instead of either you'd be better off using an underwater tool fed by Muriatic Acid.)
I found the Metal Magic via sponge treatment just as effective and far less risky. I hated doing the AA treatment.
With that said, the reason TFP typically recommends the AA treatment is that its a little more sure fire in terms of stain removal -- not everyone has the same results as I did. My reasoning for the MM via sponge test treatment was I was going to need the sequestrant anyway, so doing the sponge test did not in any way cause extra expense.
With higher calcium which I'd expect you to have with your surface, do note that large amounts of sequestrant may cause cloudiness for a few days, as calcium will also bind with the sequestrant. The other unknown to me that might be worth a call to the ProTeam tech (Kevin) would be if this method is suitable to cartridge filters.
My only other caveat is that at the time I did the treatment, I was not SWG. Regular HEDP-type sequestrants are not always as effective in high TDS water which saltwater is by virtue of the salt. Jack's Magic Purple is HEDP but is formulated to work in saltwater environments. Metal Magic does not weigh in on salt or consider it any different.
Lastly, and this is kind of moot if you're on wellwater to my mind, but all sequestrant use does over time (slowly but surely) break down into orthophosphates. This isn't an issue if you maintain TFP levels of [fc/cya][/FC/cya] -- in my 5 years of very high phosphates due to the need to treat my metal-laden well water I never once had an algae outbreak.
But if over the years your phosphates grow to record-breaking levels like mine and you switch to SWG, it is possible to create phosphate scaling in the cell...which I also did but my cell is self cleaning and still works like a champ
But knowing what I know now, and with fresh water from a liner change, my approach is modified a bit in that optimal metal management for me involves more prefiltering to avoid high concentrations and an annual treatment to control the phosphonic acid breakdown -- (via alum floc, whch is unconventional by TFP standards as floc is generally avoided and does not always work right.)
The takeaway is that its always best to treat the lowest concentration of metal possible -- sequestrant lasts longer and is more effective.
In my case, that means I use softened water on top ups (plumbed to my outdoor spigot) that then also gets further pre filtered with a 10" Pentek filter housin with a replaceable filter that filters down to 1 micron.
If your stain source is well water with a known high ppm of iron, you may wish to consider these things going forward. But if you're low-level iron municipal water that's simply built up concentration over time, dilution is always worthwhile even pre-treatment, as your results will last longer and require less sequestrant.
