As I look at my EQ, I am rolling off the highs at about 6 to 8K not 5 to 6K as I suggested you do above.
A quick thought or two. My rig (I hope to post a photo today or tomorrow) may seem like it performs a lot processing, but it is really not much more than that applied by most harp players. Most blues harp players play through a bullet mic which has a very narrow frequency response emphasizing the mid range (steep roll off at about 6K and a less step, but pretty deep, roll off as the low end is approached. (I am doing the same thing only I am using an EQ.) Most contemporary harp players then run into a delay unit to thicken their sound. Finally, they run into a 7 or 20 or 50 watt tube amp and turn the amp up to 10 so that they get tons of power amp tube distortion (at least those who seek the dirty Chicago blues harp sound). And their amps usually have a three-band EQ which they use for final tone shaping. So their sound is a long way from that of a harp played acoustically. But you have to like the Chicago blues harp sound! Little Walter comes to mind.
I am doing the above only because I am trying to capture the tonal character of a blues harp (the melodica’s cousin and a reed instrument). This may not be appropriate for other types of music.
I am rolling off the EQ at about 200 to 400 Hz on the low end and 6 to 8 KHz on the high end. Most of the musical energy from most instruments and program material is in the range between these two points. So it is not as severe as it may sound.
What it does do, is to really smooth out the highs (no harsh, squeaky, squealy sound) and tighten up and define the lowest notes on the melodica.
I look forward to getting a recording up so you and others hear it. As I said, that may take me a week or two.