Q: What do I need to get that fat tone?
A: Well, actually a good player can get a good, fat tone through any equipment, and can even produce a juicy tone without any equipment at all. It's just a matter of resonance. Open your mouth wide, relax your tongue and jaw, hold your harp well and practice, practice, practice. Of course having good equipment can improve a good tone. But for this, see under equipment -- BB
For fat tone without a mic, make your hands into a shape as though you you are trying to carry water to a thirsty friend and can't afford to spill a drop. You can only make one trip and so must carry as large a quantity as possible without any leaking out.
Got it? Good. Now do the same thing with a harp between the thumbs and pretend theat you are also trying to keep a small guppy from escaping as well as the water.
You should now have a large air chamber within your hands and when you play the harp (remember this is IMAGINARY WATER- IF YOU WERE PRACTICING WITH REAL WATER, DUMP IT OUT!) you may actually feel the air you put through the harp pushing and pulling on your hands. The more you can seal the harp off, the fatter the sound becomes. Use you face to complete the seal. The tighter you can seal the air flow, the more possibility there is for you to CONTROL THE AIRFLOW. This may seem redundant, but it isn't if you realize that it sounds different sealed and open. That is where hand wah and the like come from.
Free advice is worth the price- unless it helps you. Then it is priceless. Hope this helps you.
When you add a microphone into the equation, it lessens the space inside the cup. you also have to seal any leaks around the mic body. Other than that it is the same.
Interesting to note is that unmiced, the tight cup is quieter. Miced, it is much louder. I can double the gain before feedback by cupping tightly.
See section ~how to hold~ -- HA
That "Chicago sound" can be obtained with 1. good acoustic tone, 2. a mic that can be cupped, and 3. either a tube amp, tube preamp, or amp emulator. I've gotten great tone from a Fender Champ (silverface), Mesa Boogie V-Twin tube preamp, and my current rig, Digitech RP-100 (thanks to Richard Hunters "Early Taj Mahal" patch - thanks Richard).
If you're used to a particular rig, then change, it will take some getting used to. I played guitar directly into the PA for several years, and it took a while to get used to my Line6 POD. Previously, I used a Music Man 112RD - and of course it took a while to get used to the PA. Even different amps of the same model take some getting used to. And likewise microphones.
BTW on mics and Chicago sound, I've tried lots of 'em, and have little trouble getting a Chicago sound from any cupable mic. This includes SM-57, Green Bullet, JT-30, Shure 444 (a very good but underrated dispatchers desk mic - it's plastic, but the sound is great), various smallish electret condenser mics, etc. My favorite is the JT-30, which has a nice "natural" distortion.
BTW a JT-30 into a very hi-Z preamp/FX and the PA will give you a nice
overdriven sound as long as your acoustic tone is strong.