OK first Qs
Power wire: Yes you can connect into the same power wire, Basically making a Y make sure you have a fuse between the battery and the Y
Remote: I always go from the head unit to a toggle switch then to a fuse then to the amps, This prevents any possible shorts from getting back to the head unit. This wire can also be Y'ed to both amps.
GROUND WIRE it is VERY important to use a ground at least the same gauge as the main + wire if not bigger. Especially on high power systems.
RCA cables from the head unit to the amp or from one amp to another does nothing to the watts. My amps are set up with a line in and a line out. My RCAs go from the head unit to the first amps input then from that amps output to the other amps input and so on until all 3 are connected. The RCAs simply move the audio signal from one place to another at a very low voltage. Usually between 1.2 and 2.5 volts depending on head unit output range.
Raising the low boost measured in HZ is fine put it where you want you're bass at MID or LOW.
The input is not an adjustment, Set it at the same range as the RCA output of the head unit. Somewhere around 1.5 is a good default if the headunits output is not known. All this does is tell the amp how many VOLTS not WATTS it should be reading from the head.
LPF this switch is " LOW PASS FREQUENCIE " this is like having a built in X-over. With it on the amp only pushes the deep bass notes. If turned off the amp will push all input freq's.
Filter **** should be used to filter out distortion, I cant help you much with this one because I cant hear it :-& #036; This is how I set mine turn up the stereo to you're prefered listening volume, Come on not that loud, and adjust the filter to get the "cleanest sound"
Dont hook up two subs to an amp that is wired in mono, unless you can buy amps cheap, In mono you are telling the amp it will only have to push one sub,at a higher power, If you lie to you're amp it will live a very short life. MONO means one! Even in mono hooked up to one sub it should sound noticably different than the two subs hooked up together. If not check you're amps wiring diagram again and make sure you're wires are connected as per MFG specs.
You can also research this topic at crutchfield .com and check out the SPL DRAGS. (sound pressure level) I hope this helps let me know if you have any more Qs.