Replacing a 250W amp with a 300W amp is not going to get you any noticeable increase in output. Even if you replace it with a 500W amp, the increase will be only 3dB.
Not only that, but that active subwoofer was likely designed as a system, e.g. the amp was sized to get the best response from those drivers, and it likely was configured with some sort of active equalization as well (to extend response and control excursion below Fb). If the Klipsch amp doesn't provide the same sort of active EQ, the result might be what you're experiencing with it now.
