I wanted to create a separate thread to track this issue. This has been pointed out in a lot of the “Arc sounds tinny” posts: It seems like if you use an iPhone XS, iPhone 11 Pro, or similar modern iPhone, it results in the tuned result being extremely tinny -- very harsh trebles, almost no mids, and but the low bass (like the Sub) is still intact.
However, if I TruePlay tune using an iPhone 6/7, or a 2nd gen iPad Pro, I get much more well rounded, rich sound.
It seems like TruePlay is interacting badly with the microphones on certain Apple products. Several users here and on Reddit have noted that turning off TruePlay or re-tuning with an older Apple device results in better sound.