maybe I should have posted this in the noob section lol, but I have a SM58 and a SM57 and those work alright, but aren't giving me the good full sound I'm looking for. So are the two mics I have SDC?
No. The SM57 and SM58 are dynamic mics. A dynamic mic works by attaching a diaphragm to the a coil of wire which is moved (by the sound waves) in the field of a permanent magnet generating small amounts of electric current.
SDC stands for "small diaphragn condenser". Condenser is another type of microphone where the diaphragm moved by the sound waves is one plate of an electrical device called a capacitor which is charged by the provision of an electrical current (hence the need for phantom power on condensers). Since the condenser doesn't have to generate electricity, just modify it, they tend to be able to respond to much more subtle sounds. (There's a subset of condenser mic called an electret mic btw, but they are much closer to condenser than dynamic in how they work.)
LDC stands for "large diaphragm condenser" by the way. These tend to be those nice big studio mics like you always see in pictures. SDC mics tend to have diaphragms around a half inch in diameter and are great a producing detail. LDC mics tend to have a diaphragm around an inch (or more) across and often give a "warmer" sound while still preserving detail and accuracy.
However, dynamic mics are usually much more robust. Although they generally lack the detail a condenser can give, on some sound sources this can be a genuine advantage. For example, a lot of radio studios use dynamic mics for the cliche "announcer sound" where warmth and resonance are valued. Dynamics can generally also take much higher sound levels--for example your plan to use them on a guitar cab is an excellent use.
(By the way, a third main type of mic is a "ribbon mic" but, as we're not talking about these, I'll let you do your own Googling
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