You got me worried there for a moment because I have experimented with a number of pedal and have changed my approach a lot over that last six months. But I am back to using the pedals pictured above. I use them in a different order order however. The order I now use them in is: MXR bass compressor to the Harp Tone+ to the Harp Break (or Harp Attack), and finally to the Harp Delay. With these pedals, you can completely change the tonality and character of any melodica. If I am not looking for distortion, I leave the Harp Break (or Harp Attack) out of the mix. Lately, I am playing with a cleaner sound, but still occasionally like to dirty it up.
If you must only buy one pedal, get the Harp Delay. It rounds out the high end, taking out the harsh overtones and really thickens up the sound. I use a hot mic, so basically I use the compressor to limit the input signal to the Harp Tone+ so it does not distort.