Basically, there are a few specific tube amps for harmonica players.

Sonny Jr. Amp
Harp King (Rod Piazza mod)
Serrano Amps
Marble Amps
Holland Amps

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 atAmptone.

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 AB; 2x12-inches vintage Fender speakers; killer tone)
• Fender Bassman RI 1959 (USA; 40 Watt; 4x10-inches alnico speakers; great tone)
• Marble Max (NL; 6 Watt; class A; 1x8-inches Weber speaker; currently my favourite)

Two other amps which I am finding useful are the Pignoses:

• the little pig has a gut tone and is now my current racehorse at the rehearsals; the tone is there but you need a vocal style mic, my preferred is the Shure SM 58; with this mic I can crank the pig knob at 270°.
  
  
  
  
  
• the 40 W, professional Pignose is an enigmatic amp; at the beginning it was almost unusable; the suggested tube swapping (12AT preamp tubes) slightly improved the situation; then I reached the beautiful Steve Ahola web site and every answer was there; highly suggested for the harper who owns a solder iron.