I use a Roland Mobile Cube (six AA batteries, rechargeables are good) for my harmonica and I can get a good, clean, full sound. Reverb if I want it, a good selection of settings, very portable. There are other options. Whatever you do, take the mic/pickup that you will use into the shop with you, and you'll also need a friend with musical ears. Never buy from a shop that won't leave you alone for a while to try out the setup, and don't take the salesman's word for how you sound!