Basically, there are two specific tube amps for harmonica players.

Sonny Jr. Amp
Harp King (Rod Piazza mod)

However, there are many guitar amps that may or may not have been modified for use with a microphone in playing harmonica. Some useful information, links and a classic harp amp mini-primer can be found here.

I regularly use the following amps:

• Binson (italian brand; 10 Watt; class A; 1 10-inch vintage Fender speaker; made in the 60')
• Fender Blues Jr. (tweed coverage; 15 Watt; class A; 1 12-inch Eminence speaker; customized by Massimo Sutera - basically he turned the pre-amp schematic to the tween specifications - a lot more clean; no more gain switch, it acts as a bright switch instead; one spared 12 AX7 serving the reverb circuit !! - stand-by switch added)
• Supro (Valco) 1690 (USA vintage; 40 Watt; class A; 2x12-inches vintage Fender speakers; killer tone)