We can confirm that there is a minor problem with BitPerfect under macOS 10.14 (Mojave) which is preventing Integer Mode from being selectable. Interestingly enough, if you had previously been using Integer Mode on your selected DAC, BitPerfect was still playing using Integer Mode, even though the check box was unchecked and grayed out.
In any case, we have developed a fix for this issue. It is currently undergoing test, and will soon be released for Approval to Apple. You should be seeing it in due course as an update.