The phase control on the subwoofer changes how the driver in the sub moves relative to the other speakers. At 0, it is 'in phase' and so when the other speakers move in, the subwoofer moves in and when the other speakers move out, the subwoofer moves out. At 180, the subwoofer is 180 degrees out of phase; ie it moves in when the other speakers move out. Some subs have a variable phase control which lets you set it to anywhere between 0 and 180 degrees.
The purpose of the phase control is to tailor the sound from the sub. Sometimes if the sub is at the opposite end of the room from the main speakers, 180 degrees is best to blend the sound from the sub with the main speakers. Many times you cannot even tell a difference between the various phase settings. As a general rule, set it to the setting that produces the most bass (as long as it does not get overly boomy).
All speakers Small with 80 Hz xover is usually the best setup. The 'bass boost' will do exactly what its name implies - it will increase the level of the bass by +3 dB or +6 dB. Turning up the volume on the sub or increasing the receiver's subwoofer out level will do the same thing.
Best placement is found by trial and error but as a general rule, corners work well. Bass is non-directional because it is so low in frequency so it really won't matter if the sub is placed right-side up, upside-down, on its side, etc. But, you probably want it situated the right way up and with the woofer facing into the room. If you place it so that the woofer fires against a wall or corner, the bass response will be greater.
Given all of that - have you used the test tones to calibrate the levels of each channel including the subwoofer? Poor bass response could be caused by the levels not being equal for each channel. If you change the phase switch position, you should recalibrate as well.
