There's been many a heated argument about the alleged different "tones" of different harmonicas and the materials their combs are made from, be they plastic, wood or metal. No-one has ever convincingly demonstrated that any of it makes a scrap of difference. Tone is exclusively, or almost exclusively, the product of the players' technique, the desirable features of which are well described by BBS. Your abdomen, diaphragm, lungs, windpipe, throat, mouth and hands all contribute to the sound chamber that produces your volume and richness of tone. The harmonica is basically just twenty little flaps of metal shaking in the breeze. With a rack, your hands are not contributing. Also, the awkward design sometimes means that you're not getting the harp as snugly or controllably in the mouth as you would with hand-holding. However, rack-playing is perfectly good - if you're good!

I also think that someone with poor tone will not make it better with a mic and amp. Just louder bad tone!