AMPLIFIER GAIN SETTINGS WITH A DMM                                    
If you really want to do it right, you need to set your gains with a digital multimeter (DMM). What you're going to do is get your DMM and find the voltage thats optimal for your amplifier.
You're amp puts out 300w @ 4ohms so you're going to multiply the two together, giving you 1200w, then you're going take the square root of that, 34.64v and that's what you're going to look for on your DMM.
Next you're going to disconnect your speaker wires from the terminals on the amp. Stick the DMM leads in and then turn all your gains to zero on your amplifier. Go to your head unit and do the same. No bass boost, no nothing, set bass mid and trebel to zero.
Next, you're going to find out what 75% of your volume level is. If full tilt is 30, then you're goin to set it to 22 or 23, if it's 40 you'll set it to 30, etc.
Ok now you're going to need a CD with a 60hz sine wave set to 0db. It can be anywhere from the 50-60hz range, but I always use 60hz at 0db. YOU MUST DO THIS FROM A CD WITH THE HIGHEST FIDELITY TONE YOU CAN FIND. Doing this from an iPhone or anything else will NOT work. Why? Because science.
Ok so now you're going to play your tone and watch your DMM. Slowly adjust the gain on the amplifier until you find it's G-spot; 34.64v. This is going to give you the MAXIMUM UNCLIPPED OUTPUT from your amplifier. Next, reconnect your speaker(s) and text it out. Make sure there's no clipping or distortion, and you're good to go. It's actually a lot better to feed that sub 300w completely undistorted than 250w with even a little bit of distortion. Remember, overpowering doesn't fry voice coils, it's clipping and distortion.
Some amplifiers have 2 gain controls so treat it as two separate amplifiers. If the amplifier is 100W RMS by 4-channels for a total of 400 watts but has two gain controls, divide the total RMS by half and use that for your voltage calculations. (EX: Square Root of 200W RMS x 2 Ohms = Voltage for each gain control)

                             Subsonic, what is it and how do I adjust it ?                      A subsonic filter is basically a High Pass Filter for very low frequencies, generally 45hz or lower. That is, whatever you set it at; only frequencies above that setting go through. Anything below that setting is attenuated (made quieter).
As only frequencies higher than your setting pass its a high pass filter.
A subsonic frequency is very low, so low the human ear can barely detect it, but your body can feel it. Because you cant hear it, and they're prone to damaging woofers, filtering subsonics is a desirable characteristic in subwoofer amplifiers.
A subsonic filter is used differently in sealed enclosures than in ported.
In a sealed box: The lower the frequency, the more excursion your subwoofer exhibits in order to play it loudly and accurately. A sealed box is tuned by enclosure volume, larger enclosures tune lower, smaller enclosures tune higher. 
Subsonic frequencies potentially damage your subwoofer because they make it expend a lot of energy, travel right to the limits of its excursion and play below the enclosures tuning. 
So for safety, we want to cut those frequencies out. For a sealed enclosure you adjust the subsonic filter to 25-35 Hz, to filter the extremely low bass frequencies your woofers unable to play. 

In a ported box: You tune the port to a certain frequency, the enclosure is then capable of playing all frequencies above that tuning without an issue.  The enclosure can also play below that frequency, but only half an octave, before the cone starts over-excurting and there's potential for damage. 
So for safety, we set the subsonic to 1/2 an octave below our tuned frequency. Whats half an octave you say ? Lets do some maths ! One octave up is double the frequency, One octave down is half the frequency. 
Lets say you are tuned to 45 Hz:
- 45 Hz / 2 = 22.5 Hz (one octave lower)
- 22.5 Hz /2 = 11.25 Hz (half an octave lower)
So we take 45 - 11.25 = 33.75 Hz (rounded up to 34 Hz)
In other words, you set your sub-sonic to 34 hz. So that way, if there's music below the ports frequency, it gets filtered out protecting your woofer.
Always remember the subsonic filter is NOT a cut-off. It has a roll off "slope" where whatever frequency its set to will be attenuated, and the attenuation effect increases as the frequencies get lower. Thus the power to the woofer decreases at filtered fequencies, which reduces the excursion and risk of damage.
As a final note for the audio geeks: Subsonic filters have steep slopes such as 3rd or 4th order(18 or 24dB/Oct) this is so u can set it nice and close to your 1/2 octave frequency, or 25-35 Hz sealed,  without losing power in the neighboring frequencies.