Car Stereos: using deck to power only front speakers, watts rms, rca cables

Question
Hello Brian,
What is your opinion on running the front speakers using the power supplied by the headunit and using a 200W amp to run the rear speakers and a subwoofer? Sounds appealing to me since it would save me the trouble to use another amp for a sub in future and will be easier to wire up (i think) since all the speakers will be easily accessible from the trunk. But i am not sure if the front speakers will get drowned out by the sound from the rear ones, unless the head unit actually provides more power when it is only feeding two speakers instead of four.  The head unit i am looking at is rated at 20 watts RMS x 4 channels; if it is only connected to 2 speakers will it actually provide 40 watt RMS to these two speakers?
Here are the components i am looking at:

Head Unit: http://www.crutchfield.com/p_020D385USB/Clarion-DXZ385USB.html?search=Clarion+DX...

Front Speakers: http://www.crutchfield.com/p_065S55/Boston-Acoustics-S55.html?search=Bostin+Acou...

Rear Speakers: http://www.crutchfield.com/p_065S95/Boston-Acoustics-S95.html?search=Boston+Acou...

Amp: http://www.crutchfield.com/p_500MRPF300/Alpine-MRP-F300.html

Thanks again.

Answer
Hi Nadeem,

There's nothing wrong with using a 4-channel amp to run one pair of speakers together with a subwoofer, but I'd strongly recommend making one change to the plan:  use the amp to run your front speakers, not your rears.

The front speakers are much more important to your overall sound quality than the rear speakers.  If you're only going to amplify one pair, the fronts are the ones to choose.  I understand that you're hoping to make the wiring and installation easier, but it's worth the extra work; and it won't really make things much harder.  As you're going to be running RCA cables and a remote wire from the head unit to the amplifier location in any case, you can just bundle the extra speaker wiring along with those.  (Also, I erred slightly in an earlier answer to you: you won't have to cut any factory speaker wiring to make your connections, because you're installing an after-market deck and using an adapter harness.  You can just connect the speaker wires on the harness to the speaker wiring coming from the amp, instead of connecting them to the speaker wiring coming from the head unit.  This eliminates the need to cut factory wires).

One other advantage: the amplifier has a built-in crossover circuit that allows you to cut the deep bass frequencies out of the speaker sound.  The smaller front speakers will get more benefit from this feature than the 6x9" rear speakers.

You might want to consider using a 6.5" speakers instead of the 5.25" speaker you've chosen for the front doors.  Either size will work, but in either case you'll probably need to use a 1/2" spacer bracket to mount the new speaker.  This is because of the odd-shaped cutout in the door metal for the factory speaker assembly.

When you use only two channels of the head unit outputs, it does not increase the power output; the head unit is still limited to 20 watts per channel.

I once wrote up a guide for removing the factory radio in the 2001-2005 Civic.  Here's a link to it, in case you might find it helpful:

http://kzcarfi.blogspot.com/2008/09/2001-2005-honda-civic-dash-disassembly.html

Hope this helps!

Brian