I am not sure on the amp. There are several companies that make amps specifically for harp players with tons of tubes inside for that warm sound. One company is called Harp Gear. I like what I am reading on the Harp Gear website. Many harp players also use Fender guitar amps like the Bandmaster, Twin Reverb, and Superchamp. It seems the older and more beat up the amp, the better it is!
I was thinking of going to a Guitar Center with my melodica, mic, and stomp box and auditioning a few Fender guitar amps to see if they can deliver the volume and sound that I want before buying. I was also going to maybe ask Harp Gear if I could pay for shipping and try out a loaner they may have available.
Harp players reaching for the Chicago blues harp sound can make a 35-watt amp fill a club because they are cupping (air tight), compressing, and over-driving the mic element and setting the amp on 10 for lots of distortion. Playing acoustically (no air tight cupping of the mic) like we must do with melodicas changes everything. My 125-watt monitors can barely play loud enough for me to hear myself over the sound of the instrument, and I am always on the verge of feedback.
You should check out Lone Wolf Blues Company. Then make a Harp Attack that is a tube-drive stomp box. It works awesome, but I sent mine back only because I was not sure if I wanted a distorted sound. I may have sent it back prematurely. It is a tube preamp amp in a stomp box. Yet, I always come back to a clean acoustic sound for the melodica. A number of harp players also play acoustically, again without cupping the mic for distortion, so a clean sounds can work well too.
There are hundreds of websites for harp players. Only one for melodica players!
But start at ModernBluesHarmonica.com. Tons of great information and dozens of links to informative websites.
So there you have it. The quest continues for the ultimate blues melodica sound.