When you wire a 4 and an 8 in parralell,technically(not taking into account the impedence curves,meaning the woofers DO NOT have the same impedence onver the freq spectrum..) the 8 ohm would see very little actual power..it would get something,but not what you would expect.. electricity takes the path of least resistance..
the impedence curves are what allows some power to get to the 8..as far as the seperation,that would be better more for the bracing qualities,rather than the seperation,just remember if you put two woofs in a box,do not port it based on the total volume,port it based on the volume each woofer has..like 2-1 ft3 boxes(but only single chamber) use double the port for a 1,rather than the port for a 2..Make sense? but with 10's,you should run any less than 1.25 ft3 per woof.any less seal it,or do an aperiodic,for real spl,with real lows,do @ least 1.5,pref. 2 ft3. like going with a jl 10" w7,2 cubes,you can port those babies down to less than 30! and still feed them with 500+ watts.I know they say new woofs can go into smaller boxes,but if you tune that low,say in a 1.25,then the woof will not handle even 1/2 as much power.. thats why the overbuilt the crap out of that woof..dont waste it! if you only have 2 ft3 to work with,dont buy a 12,a 10 properly done in 2 ft3,will be LOUDER,and sound BETTER than a 12 in the same situation!
as a guide: 8" 1 ft3(i cant tell you how many hundreds of pairs of 8's i have out there that people guess are 12's all day long,i cant put more than 100 watts(SUPER CLEAN) to them,each,but the wattage guesses range from 400 watts,to 600..)these are based on people that hear underboxed woofs all the time..
anyway..
8's 1 ft3
10's 2 ft3
12's 3.5 ft3
15's 6 ft3 (min) 10-12 optimally)
i know most of you think thats outrageous, but i am sure most of you havent heard these sizes,properly tuned.
I ONLY do sealed/small for a very few people.
my 10's look like 8's because of the size of the box,but sound like the better than average 15,with a fraction of the power..
and dont waste you money on super woofs,unless you get a super amp..untilyou max out a particular woof with power,it is highly unlikely you could hear the diff between a 200 dollar woof,and a 500 dollar woof.most people cant hear a 30 hz wave in the average car anyway! most of the stuff peole think is bass is around 50! 30 just destroys the car! try a subsonic @ the res freq,and tune the box @ that as well,but if the box is smaller,tune it higher,like if you have a 10 in 1.25,for a decent woof,tune it to like 35-40,susonic it @ that freq,and you wouldnt believe what it will do.
that is long enough i think,sorry..i am long winded,but i am passionate about this stuff,ocd passionate,and have 16 years of experience to relay..
'Stereo' Mike
The Audio CARtist,LLC
9600 Balt. Ave.
College Park,MD 20740
301-474-6260