Here's what I get from the OP's posts...
He is measuring the
water's PH when identifying needed adjustments.
He is using the acidic wort to "clean" his copper coil.
The reality is that water PH has nothing to do with mash PH. You need to know your water mineral profile in order to make accurate adjustments.
I've found that taking measurements 20 minutes in to a brew will tell you if you are off, but by then it's too late.
If you know the starting water profile, you can enter those values into an on-line calculator (I like Brewer's Friend) along with your grain bill, and it will very accurately predict your PH level. You can use your PH meter to check it, and make adjustments to your recipe for next time.
Regarding the dissolved copper, that's just bad all around, especially if there is any green Verdigris present:
https://en.wikipedia.org/wiki/Copper_toxicity
Like others have said, tannin extraction occurs when both the temperature is too high, and the PH is off.
Why even do a mash out? To me, it's a waist of time.