You are looking at it a little off. in all actuality, the voltage difference in the rca's stops at/near the gain pot. in a controlled environment, aka "on paper", there would be no real difference between a .2, and an 8v signal, given the gain is adjusted accordingly. where it comes in to play, is that there is almost always going to be noise in the signal line, which is usually a low level. the higher your base signal voltage is, the less voltage sensitive you can make the amp. in short, the signal range you are using vs the noise present/picked-up in the rcas gives you a higher signal to noise ratio with higher voltages. this is different than the SNR rating of an amp or head.
are you open to other brands? how deep do you plan on going into your system? perhaps you should put off the head unit until you do more research and are confident in your own choice.
in my experience, both are just as bad, and i believe some even use the same laser/pickup unit inside. it all depends on the model you are looking at.
you are back-wards there buddy. poly-fill basically has 2 effects, and one is to fudge the box to "look" bigger to the driver
actually, both the alpine and kenwood put out the same power. one option that alpine does offer, is the "module" amp that plugs into the back and is actually an amplifier, so you do really get more power from an alpine via "plug-n-play"..... however, you can take that alpine amp unit and simply cut the plug off and hook it to any head unit with speaker leads. i've played with both alpine and kenwood since the 90's. and my opiniions compare the units on parallel model levels. it sounds like you are pitting your high-end alpine units against the lower and mid kenwood units. i will say that neither are my top picks for brand.
the short list of both brands inheirent average known problems: kenwood: face ribbon failures, and sometimes lacking in processing, where you would think it should be. alpine: one constant with most alpines is a horrible base-line sound reproduction, which i would describe as being a stale sound that takes a lot of tweaking to smooth out. problems i have see with both: cd mechanism issues. i have seen both in any level of head unit having a motorized face snap discs in half. i've played with a bunch of kenwoods that take several tries to get the disc in or out, but you usually still can get it, just a pita. i actually have a stack of 5 alpine units (basically the same unit) that have mechinism problems that cannot function at all, and a few other alpines similar issues, even higher end models. the kenwood was the first i experienced snapping discs, and that was basically the highest model. so, basically, i see them about equal in mechanism problems. given the choice, i would take the kenwood for out of the box sound, but for a general suggestion in head units, most would be happiest with pioneer. they have always been a safe bet for general use. the buttons can be cheezy/flimsy on some models, but so many more people have been happiest with them in the general public. my favorite heads in the last decade are eclipse, but that is me.
honestly, if your head unit is working, i would go right to the box first. the subs and amp are nice equipment to build off of. the saying goes "10%equipment, 90%install makes the system" so, my main concern is to make sure you can feed the amp enough current, (lack of low-end bass at higher volumes could be that the amp is clipping) and that the box is tuned to give what you want. also, make sure your amp is tuned decent.
