It sounds like you have the right cap, you're just overpowering it. What kind of amp is it? And remember a cap is only a 6 db slope crossover, so frequencies quite a bit below the crossover point may still be slightly audible. If the amp has a high-pass x-over in it you should be using that.
Also keep in mind you're not working with a high-quality product here - the results are only going to get so good before you've hit the limits of what it can do.