Mic'ing the cabinet with an off-axis setup does help shave off some of the high frequencies and gives a somewhat thicker sound.
A larger cabinet with more/larger speakers produces a heavier, fuller sound than a smaller cabinet with a single speaker. Typical blues/clean sounds come from combo amps, usually one or two 10-12" speakers, while rock/metal sounds come from amp stacks, which is an amp head and 4x12" speaker cabinets.
Tweed is a term used when talking about vintage gear that was often covered in tweed. When talking about amps, it usually refers to vintage Fender amps used in blues and country.
Amps come in lots of flavors. Even amps that are very similar have lots of differences. For example, a Mesa Rectifier is an amp that is capable of a ridiculous amount of gain and has a very heavy, percussive tone with tons of low end. A Marshall JCM2000 is also capable of very high gain, but the tone it produces is very different. It's much brighter and has more bite with a very tight low end. Neither is better than the other. They just cater to different tastes.
In a guitar amp, tube or otherwise, the preamp feeds into the power amp which powers the speakers. For a clean sound, you'd turn down the gain/drive on the preamp, not take the preamp out, as the point of the preamp is to color the tone of your guitar.
For special effects, you might want to use some of the dry signal. Provides a harsher, very abrasive tone.
I almost always have the tone controls on my guitars set at 10. Experiment, though, and see what works for you. Rolling back on the tone knobs produces a very woman-like tone (think Eric Clapton) and is useful when using very bright pickups, such as a single coil pickup in the bridge position.
Ooooh, sorry about that. I must have left out a folder. I'll have to check my files.