If you don't deaden the exterior door skins, you will certainly be lacking in midbass response at some frequencies, due to the outer door skin vibrating out of phase with your speaker.
How did they deaden the interior door skin? Is it sealed off? How much did they install, in sq foot? What about the plastic door panels, anything done to those? The plastic door panels in my car were the worst offenders.
In my opinion, the minimum deadening you need is at least 15 sq foot per door. I used more. About 5sq foot on the outside door skin, another 5-7sqfoot on inside door skin, then in my case I put about 10sqfoot on the inside of the plastic door panel. Then I glued sound dampening foam to the inside of the plastic door panel to avoid vibrations of the panel against the door metal. You may find differing opinions on this tuff here - be careful about the (extremely prevalent here) bassheads telling you things that don't jive with your expectations of a coherent sound stage and (relatively) flat frequency response. Deadening your trunk area is important due to localized vibrations allowing you to tell where the sub freqs are coming from too; did they deaden your trunk? If so, how much deadening, and installed where? Too many shops are used to making cars designed around the subwoofer instead of the entire soundstage, and many of them seem to think that deadening the trunk roof and the metal door panels that the speakers are bolted to is 'all that's needed'. You don't need to do 'the whole vehicle'. I consider that taking out all the carpeting, roof liner, etc. etc. and literally coating the entire vehicle in deadener. It's overkill and you reach a point of massively diminishing returns very quickly. However, it is quick and easy to do the large plastic panels, at least, and the rear deck where the sub has the most impact, and those are what make the biggest difference.
You can judge, to an extent, if you need more deadening, though the human ear isn't the best device for this and many times you get more of an idea of 'it doesn't sound right' when dealing with frequency response 'holes' than a particular idea of, 'oh, why didn't I hear from 80 to 95 hz in that bass sweep?'. At least you can find if you have major vibration problems. Find a track with no vocals, and no high-hats or other midrange sound - a pure midbass/bass track. Play it at a reasonably high volume, then move around the cabin, listening for vibrations. You may find that with earplugs half in your ears you can hear the vibrations easier. Vibrations at high frequencies are what you want to avoid. A low frequency vibration will make your bass sound a little 'muddier', but won't signal your ear the same way a high frequency buzz would do. After I deadened all the plastic in my car, I was able to play the system louder before noticeable vibrations were introduced, thus the sound is 'cleaner' to the ear, especially at low volumes. I realised that a lot of the problems I was experiencing was due to additional sound being created through vibrations of the car's interior panels. Now, you're pushing a single 10" so you might not be getting THAT loud, but I am sure there will be an effect.
You'll want some content that varies in frequency - my panels vibrated really badly around 90hz, but not so badly at lower and higher frequencies. As I added deadening, they vibrated at different frequencies (and not at all at frequencies where they used to). If you can hear buzzing coming from your car that isn't being created by a speaker, then you need deadening. You can usually tell where it's needed simply by putting heavy pressure on the area that is buzzing.
Honestly, unless you are planning on going fully active, you can do fine with 2 amps. For example, that arc 300.4 could be bridged to push your front speakers (through passive x-overs) and then a monoblock could power your subs. That's how I did it at first - my 4150xxk is rated approximately the same as your arc 300.4. I bridged it to the power line CS and was pushing ~340watts per side. The only reason I added the 2100XXK was so that I could time-align my tweeters independently of the front woofers; I noticed that midrange sounds would 'walk' back and forth juuuuuuust slightly as the frequencies raised through the crossover frequency band - the tweeters would take over, and the difference in distance between the tweeters and me is more extreme than the difference in distance between me and the woofers. Independent time alignment per channel took care of that, but adding a third amp and tuning it is honestly a PITA. It took a year or so for the 'wandering' to bother me enough to motivate me to add the third amp. You're still fighting the subwoofer sounding like it's behind the car problem, so i don't think you'll be needing a full active setup quite yet //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif Tuning a fully active setup can be a real investment in time.
Dealing with 3 amps isn't worth the hassle unless you have processing for all channels and intend to run 6 independent channels (or at least 5 - 2xtweet, 2xwoofer, 1x sub), in my opinion. I would spend the money re: your rear coaxials on better front speakers and on higher rated amplifiers before i'd add coaxials to the car. You've really got to hear a set of front speakers that are designed to get loud to understand - rear speakers don't help unless you have a shitty frontstage, and if they do help, your frontstage must be really bad and the money would be better spent fixing the frontstage so you have a good sound and correct imaging rather than trying to band-aid it with more speakers in the back. Inevitably you realize that your imaging is ****ed and, due to speaker distance differences, time delay, reflections and phase issues, you STILL won't have dealt with the original problem - the front stage not being up to your expectations.
Wait; you said your 'amp' is a ks 300.4. Does that mean you're pushing 2 both fronts and rears with that amp now? I guess the D8400M is your sub amp then? Or are you using 2 channels from the arc for your sub now? Honestly, 180w is decent for the BA pro60's, but I have read of many people pushing double that to them and being happy with them, and read many posts where people swear that they need 300w to really get moving. My power line CS's were 'decent' at 150watts, but 'awesome' at 340watts+ available (right, filtor1? //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
Aren't you pushing 400w/side to yours? lol). You need some headroom so that you're not maxing the amp out, even under the heaviest loads to get a good dynamic response without clipping.
