RValenz - welcome to the HDTimeline Forum and your first post...while others will weigh in shortly, I would say your tread mileage is based primarily on how you ride and use your bike. The K591 is supposed to be a softer compound...read softer means better traction and higher performance at the expense of a few thousand less miles. The D401's are a touring/cruiser bike to be a harder compound...read harder means less traction and slightly less performance but consequently better mileage, closer to the original mileage your Streetbob enjoyed when newly minted in 2005-2006 (probably why you got over 10,000 miles).
The real question, did they perform as you liked...many HD riders are happy with the stock Dunlops...they are a fine balance between performance and mileage. Use the Search tab and key in tire poll and see what your fellow riders are choosing. Enjoy the new tires...what ever you decide (if your front is near to changing), go ahead and use matching front tire - branding, part number and application.