It's been my experience that most communications grade speech processors leave noticeable artifacts behind in the audio. They usually have limited bandwidth that ruins any Hi-Fi attempts. Often having the unnatural sound heard in many TV commercials although, not as loud as them.
The type of filtering used after the clipping makes a big difference here. Some appear to have no filtering. Better sounding boards use active low pass filters to provide a wider bandwidth while still having a sharp cutoff frequency to remove harmonics from the clipped waveform.
If you want to bring the majority of your audio content towards 100% modulation and still sound clean, this link will get you on track:
COMPRESSOR LIMITER FOR AM TRANSMITTER BROADCAST - jpl995
The board shown here is assembled and relatively inexpensive at around $40 shipped. It's designed to pass broadcast fidelity and can be set to give more compression and clipping than you'll ever need. Set conservatively, it's not even detectable to the ear until you bypass it and the average modulation level drops below 70% as the peaks hit 100%.
One last thing to consider is this type of board works with line level audio. That means you'll either have to use a mic with a strong pre-amp or insert the board in the existing audio chain after the internal pre-amp stages. If you don't have enough drive to the board, it's useless because it won't go into clipping.