What we don't know, and what Pass is unlikely to share with anyone, is just how many of each model has been sold in the marketplace. How many units are sold new often dictates how many are available on the used market more than buyers being satisfied or not.
Also note that the Xa160.8 is a pure class A amp while the X260.8 is an AB unit. It could well be that, if one buys fully into the Pass philosphy, that a class A unit is the greater preference of buyers.
So, there is a lot we don't know which means that our speculations are little more than naval gazing.