Thanks. It seems--at least with the WAX206--160 MHz works. Though there are reports of AC mode working better than AX when going through walls or with Apple devices. Particularly at higher connection speeds. 160 MHz operation appears to be a "same room" use-case--which is better served with multiple APs than a router hidden away in a closet.
As for the extended channels required to use the new 160 MHz allocation: I was asking if they can be enabled or not. I can't find anything about the MT7915E in regards to this. I though all AX radios would support the extended 5 GHz spectrum provided by U-NII-3/4 and U-NII-4, but it looks like the MT7915E doesn't. For such a low cost product, I guess this is fine. But I was hoping that I could use these new channels. Oh well.