I've heard that Rod Piazza said that the harmonica should be the loudest instrument in the band. Well, it ain't gonna happen. Guitar players have an edge over harp players. They can crank their axes up to 11 and not feed back. Old tube amps sound good cranked, but if you hook up a harp microphone to a tube amp it screams like a banshee when you set the volume over 3. But great Harp Amps are simple things without much in the way of preamp stages and usually have a lot less feedback.
Now the question that I get asked over and over again is "What is the best harp amp?" Unfortunately the answer is always "It depends".
If you play Chicago style blues and are looking for the tubey overdrive sound of a vintage Fender Tweed, the answer is one thing. If you are looking for a modern jazzy sound with clear horn like tones, then the answer is something else. Folk and Country each have distinct characteristic harp sounds and you'd play through a different amp for each of them.
I listen to Big and Little Walter, Kim Wilson, William Clarke and others. I like the big Chicago Blues sound that they get. I also like Sonny Boy, Phil Wiggins, Slim Harpo and other players with a more natural sound. I want a harp amp that will play overdriven and distorted when I need it to, but I often play in a more laid back style.
My point is that my amp needs are unique. You are invited to disagree with everything that I say here. Your amp experiences are likely to be different from mine. My only motivation here is to lay out some ideas that I have about amps and I hope this would act as starting point for you to develop your own opinions about amps.