Troy, thanks for the link to the harp/microphone website. Wow, what a host of information. Just what I have been looking for as I am only now getting started with amplification. Melodica-Me, that looks like the contact mic to get if one is to use a contact mic. It looks very high end.
Steve, to answer your question, one way of minimizing the key click is to roll off the high end frequencies using an EQ. And of course, play the the keys more smoothly and softly if possible. I like to pound on the keys due to my organ playing background, so trying to trigger the keys smoothly is a big adjustment for me.
Here is what I am doing now for amplification. I am using an SM 57 mic running into a Presonus RC 500 preamp/channel strip (1 rack space). This is a solid state, very clean preamp that includes compressor and EQ sections. I only received this preamp a few days ago, but it is blowing my mind. It is just what I wanted and enables me to really shape my sound. (I am returning a Summit high-end tube preamp that while very good, just did not do it for me. I think I am preferring a clean sound for now, but I definitely am going to explore a gritty over-driven harp type sound when I can.)
My signal passes from the Presonus preamp/channel strip to a Lexicon MPX-1 multi-effects processor where I am experimenting with delay, reverb, and many other effects such as pitch, modulation, EQ, etc.
From the multi-effect processor, I run the signal through a Rane 31-band EQ and really squash the high and low frequencies (which I am also doing on the Presonus channel strip). All this squashing of the high and low frequencies gives me a harmonica tonality (like having a bullet mic) but I retain a very clean sound.
I use a JBL full-frequency two-way powered monitor (10-inch and a horn) to amplify the sound.
This is my starting point, and I am liking it a lot. The tone is a smash up of accordion, harp, and saxophone. And I can play the melodica in all of these roles as I play along with blues and roots music.