when will we see a X1 power book or a significant discount on the old stock of x1 power books?
In the old days with a popular publication you would find over-stock on the remainder tables at the big booksellers a few months after the first rush of sales had ended. A specialty book like Sonar Power never gets the kind of distribution to justify printing a lot on a first run, or unsold returns shipped back from hundreds of little book stores. If there are a significant number of copies left, some of the stores (or individuals) that are stuck with them may list them on Amazon at a cheaper price after they start receiving the new version. Until they are forced into awareness that the old version is obsolete, there will be little interest in giving discounts.
With e-books and print on demand, there will be very little incentive to discount old versions of books. They will become like old versions of software-hard to find and even harder to price. Your best buy will be on a used copy some months after the new version is distributed, as people who have bought the old copy replace it. But by then the price they can expect may be so low that it is not worth selling it to most readers.