Phase will not take out bass at all..
Weather its a dial knob with adjustable phase from 0 to 180 degrees, or just a switch that you can either select 0 or 180 degrees it does the same thing.
It changes the polarity of the sub(s) being ran off that amp.
If you have 2 subs, and 1 amp per sub, and flip the phase on one amp and not the other, one sub will move outward when the bass hits, while the other will move inward. The subs firing 2 different directions will cause cancellation. Cancellation will make it sound like there is no bass.
If you have 1 or 2 subs running off one amp, and flip the phase, the subs will both move in the opposite direction at the same time, while having no cancellation.
If your amps are strapable (bridging 2 amps together) you will need to have your master amp set at 0 degrees phase, while your slave amp set at 180 degrees phase. You do this because when strapping any amp you use the positive speaker connections on each amp for your speakers positive and negative, and then connect the speaker outputs negatives on the amp together.
Amps that have the adjustable phase knob are a tad bit better for strapping amps because you can tune it for the most output of the amps.
Phase is also best used when your running your subs inverted (having the magnets showing, and the speaker cone inside the box), this way you do not have to connect the speakerwire backwords on the amp or sub, you just simply flip a switch.