When I think of a tube amp, I mostly think of overdriving the preamp amp stage and maybe even the power amp stage to really get some grit or breakup or significant distortion; but you don’t have to over drive a tube amp that much. You can just overdrive it a bit to get a warmer tone.
Many many harp players (using a mic) use 5- to 15-watt tube amps and turn everything up to get significant warmth and distortion. There are several companies that make small tube amps just for harp (Harp Gear and Mission Harp Amps are two of the companies).
Of course, as you know, Fender makes many tube amps that potentially could work very well.
I will no doubt try a tube amp one of these days. I have played through two vintage Fender Champs (one of the holy Grails for harp players) for a few minutes but not long enough for a good evaluation. But I can tell you they did not jump out and scream “buy me.” But I would needs several hours to complete a really good evaluation. I had 5 minutes.
I am getting great results using alto melodicas and running them through an SM57 mic, the harp delay and harp distortion pedals that I have, and a solid state, but very-high-fidelity, JBL, bi-amped, 2-way, 10-inch powered monitor. While the monitor is super clean with a flat frequency response, the harp delay pedal is essential at rounding off the high end, adding low end, and thicking the sound with delay. The difference is like night and day. The delay pedal does not even have a tone knob. These EQ characteristics are built right in to the pedal. This is the EQ curve that harp player like.
Then I run into the “Harp Break” pedal, which has a triode-tube like component I believe. This pedal enables me to add realistic tube overdrive, and has a tone control for adding gobs of bass. When I have these pedals setup to get great tone with my melodicas, my voice nearly unintelligible through the mic because it is so bassy.
For about $170, you can try the harp delay pedal, and always send it back if it does not do the trick. That is a lot cheaper that getting a tube amp. I doubt you will send this pedal back. It is awesome.
If you want more bass and you want to add distortion, then add the Harp Break pedal.
So far, here is the trick from my experience to get a good tone and get sufficient volume from a near-field monitor with minimal feedback… minimize effect pedal use and do not use really hot mics. I went back to the tired and true SM57 and stopped using the compressor pedal. I now only use two pedals (delay and tube distortion). I ensure my gain structure is good. I don’t get the wicked distorted sound that is in a couple of my demo recordings, but I can get just the right amount of breakup when I want using the right meoldicas and good mic technique. When I don’t want distortion or a sound with more presence, I turn off the Harp Break pedal, and can get a bright (but still rounded) sound for cutting through the mix or playing passages in the upper registers.
Less is more. I keep learning this lesson over and over in my exploration of how to amplify the melodica.