As you turn up your amp, the way it reacts with the cabinet holding the driver(s) as well as the room itself changes. This often results with the sound getting very different as you crank it. Sometimes you will get more bass and sometimes less. This is because the amplifier as a whole is now exciting different areas of pressure in the room, called room modes. The closer you get to a room boundary, the more air pressure builds up, thus more bass. This is called corner loading a speaker. In order to get the benefits of doing this, you have to be in a similar zone, ie, against the opposite wall boundary.
Most small cabinets get bass by cheating physics. When you turn them up, the cabinet has a different set of frequencies bouncing arund, and most of the time small cabs are voiced to drop the bass when you crank them to avoid them walking away on you, or falling apart!
