Sorry Stavros, there is no way to make a cheap FM-exciter behave, no matter how much you EQ the signal. Believe me, I’ve tried.
If you care at all about getting a loud, powerful signal, the pre-requisite is to have an exciter that can pass 1 through 60 kHz flat. If the exciter has inferior performance, no audio processor in the world will help you.
On the other hand, you only need to replace the exciter, which is cheap. You can still use the same power amplifier, antenna, etc.
I recommend the Broadcast Warehouse PLL+ 1w. Impeccable performance, and it’s under 200 dollars.
Best,
///Leif