If you use an amplifier, you can use a filter before AND after the amp to be as clean as possible. You don't want to waste power amplifying RF that you are going to throw away anyway. The amplifier it self can actually create harmonics and oscillations we call "birdies". So, a filter after the amp is vital.
Don't over modulate. If the meter shows you're at %105, trust it. Unless you spent thousands of dollars on audio processing, don't expect to sound as loud and strong as the commercial stations. To get that loud on the dial, you'd probably have to deviate (modulate) way past the channel edges and bleed into adjacent channels. Don't presume that peaking and tweaking is making your amp more powerful. If you go messing with the tuning of your amp or filter, you could be creating oscillations or non-linearities that are introducing AM noise into your signal. Your meter might show more power, but it could be that the extra 2 watts is a horrible whilstle being blasted into an aircraft band.
AM amplifiers need to be linear. If you're pushing your amp into clipping or into a non-linear area, you're are probably splattering all over the dial. With AM, you also need to be concerned about the harmonics below your carrier. AM puts the RF in side bands above and below the carrier. So, if you're clipping your amp and creating crap you can't even hear, it's creating side bands above and below your channel. In the AM spectrum, Your inaudible noise 20kHz below your carrier could be squashing some guys coast-to-coast reception, 50 miles away.
Beyond that, use the least power possible. Find a higher gain antenna and reduce your power. Coil up any extra feed line coax so it will act as a choke. Look at ham web sites about coax chokes.