What kind of DB pass have you got? I did see you can get Flix tickets from Stuttgart airport to Cologne airport and back for a reasonable price, don't know what getting to and from the airports is like and the ones I saw were very early in the morning. Also some from Kornwestheim near Stuttgart. Don't know if splitting train tickets would be cheaper, say Stuttgart -Frankfurt-Cologne, or if you could do the whole journey by connecting regional trains. I see there are still loads of the match ticket holders discounted tickets going, maybe worth waiting to see if the Fans Pass is actually checked or if you could get away with a screenshot.