Did the howl start immediately after replacing the pads? or after a little bit? If it was delayed, it could be the issue described in this article:
http://blueliquidlabs.com/repairs/solving-that-brake-squeal/
I would definitely check the rotor thickness if you haven't already, also try different rotors/sizes of rotors, swap front to back etc, just to try to isolate the issue.
Also, I've found the most reliable clean method for both rotors and pads is to pour some isopropyl alcohol on them and then light it on fire and let it burn off.
I would also clean the caliper with isopropyl alcohol, someone mentioned issues with the seals, I haven't had any issues personally, but your mileage may vary. The calipers have little pockets in them which can hold mineral oil after bleeding using the Marshy method, I've found isopropyl to be a good way to ensure no oil remains.