Called Proform tech, who forwarded me to Quick Fuel Technologies. The tech guy there was very accomodating, and explained to me why carb backfires happen.
(As probably everyone else on this forum besides me knows) carb backfires are the result of a lean idle condition, in this case brought on by the increase in airflow through the proform body. His take was that given that the motor was running fine with the old 650dp, the problem wasn't a case of a timing issue. Since the dp doesn't have 4-corner idle adjustment, the proform is sending more air into the secondary idle circuit. There's a quick test that I'll do that involves changing the rear air bleeds around, and I'll also make the primary circuit a bit richer.
Updates to follow (for those that are following this breathlessly
)
JLW