I would stick to fixed heads but I also prefer a fixed blade setup regardless what KE is produced. There are many new fixed blade options out there that are suppose to fly with your field tips regardless if your bow is tuned well or not. However, you owe it to yourself to tune your bow. Not only will you shoot better but your arows will hit straight and true allowing you to get max. penetration and accuracy. I suggest you give a strong look at the new Nitron, Crossfire, Slick Trick (very accurate!) and other short ferrule compact head designs. If you have your heart set on using a mechanical go with the Rocket Steelhead or Steelhead XP. Here is Rocket's website:
http://www.rocketaerohead.com/. However, they recommend 50 or 55ft/lbs of KE I believe, to ensure that the blades will open. Their "Ultimate Steel" fixed blade is very good also and flies like with your field tips as well. I hear good things about the Vortex expandables as well. Whichever you chose you will have better chances of blade deployment if you keep your mechanical broadhead size to 1 1/4" or less (expanded blade diameter).