[Amps] problem with Low pass filter and ouput power

[sasas asasas]

Hi. I have a pll for 160m band and a linear amplifier with mosfets 10w maximum r.f power.PLL's output is only 400mV p-p.  I have construct a low pass filter with standar compnents which is calculated to cut off above the 160m band frecuencies. The PLL and linear 10w are broadband. I have adjust the linear to give 2w r.f power at 160m band. I saw at swr-power meter that the linear gives, at lower frequencies (frecuencies 200-400khz above 160m band frecuencies) 4w, at the same voltage with the filter connected, and 2w at the same frequencies without the filter connected. Why?
It's weird that the low pass filter increase +2w the r.f power. do you have an explanation??
My goal is to increase the power at the 160m band. I have a suspect that the filter cut off some useful power. Thank you