The only DC that gets through is through the resistor. In many super high gain preamps you'll find very small bypass caps attached to the plate resistors. Capacitors are also sometimes used in parallel with resistors in the signal path. In Fender amps, you'll often see capacitors with a switch placed in parallel with the volume control. Unless the volume is flat out, high frequencies get preferential treatment at getting to the grid of the next gain stage.
Lower frequencies can reach the grid only through the resistor.
Many Marshalls use bypass caps around both volume controls and fixed resistors between gain stages, though these generally don't have a switch to bring them in and out of the circuit. Most EQ circuits are just complex arrangements of resistors and capacitors. If you wanted a thicker, bassier tone you could replace the .027uF cap with a .047uF or .1uF cap. You could do the same thing to coupling caps C2 and C3. You could put a capacitor in parallel around the volume control.