I can't see how it can be the filter that's causing the slow ammonia drop. I have exactly the same and my ammonia dropping was fine.
I thought it was never going to happened - but eventually did :] this fishless cycling is a test of patience. But just think at the end of the day - what would you sooner have?
A cycled tank with HAPPY, HEALTHY fish or fish that have been exposed to toxins un-necessarily!? Now trust me I'm not the most patient person in the world always wanting soon as, but I managed to near the end of my cycle (bar the minor complication with nitrite) it will happen, just have a little bit of faith, and in the meantime we're all here to support you after all don't forget we all have something in common :]