THX multimedia certification doesn't mean much. The true THX certification requires a crossover point of 80hz. The Z-2300s run at 150hz crossover so a lot of midbass will bleed through.
At head-fi the users have roughly defined boominess being a peak at the 125hz range.http://www.head-fi.org/forums/f4/describing-sound-glossary-220770/
The subwoofer will definitely drown out the satellites, although this may be alleviated with putting the sub in the middle of the room. When a sub is dead center in the room it will cause what is called a "bass null" since at the listening position output from the driver is being cancelled out by comb filtering from the reflection from the far wall.