I have the Pioneer 919, and had a lot of fun getting the best out of it. Took me a lot of time, and a lot of forums!!
I would first suggest making sure that your sub's crossover (at the sub!) is set at maximum - otherwise you are going to be creating a hole at your crossover frequency.
I also have my tower fronts set with a 100hz crossover - to me this gives the most balanced sound. I have checked with SPL meter and I had a large dip when crossed over at the (usually) recommended 80hz - trust me, whatever sounds best to you IS best.
Its also possible that your sub is set at the incorrect phase setting - if the sub is out of phase that can result in what sounds like "inconsistent" bass, which makes your sub sound weak in certain frequencies - most likely around your crossover!
The best way to check for this is to play a test tone (get them here at http://www.realtraps.com/test-cd.htm
) at your selected crossover frequency and put your head between your subwoofer and your front speaker whilst changing your sub's phase switch settings - whichever setting results in louder sound output is the right one!
Good luck and have fun