New Alpine - poor bass response

Question

I am hoping someone out there has a recommendation. I haven't been satisfied with the answers I have gotten from either the local Best Buy store or Crutchfield, whom everyone seems to rave about.
Thinking it would be a permanent solution I replaced the stock - and mediocre - stereo in my 95 Nissan pickup with an Alpine CDA-9883 deck and Sony xplod 4 way poly cone 6.5s (rear) and 5.25s (front) (XS1342 I think). Installation was basically a snap.
I noticed the difference in the high end immediately - very clear - but I was quite underwhelmed by the low end, which I have since compared with the stock stereos in both our 2001 Toyota Rav4 and 2000 Ford Taurus. Both of the latter sound warmer and have much rounder tones at the low end even with the treble set flat. I was not expecting the shake the neighborhood but I don't understand why the Toyota and Ford factory stereos should outperform an Alpine!
I have experimented with the bass bandwidth and center frequency settings and found little audible difference between them.
I find it hard to believe that the deck is the problem but can the speakers really be that bad? The salesman at Best Buy, where I bought the deck, suggested that the polycarbonate speakers often underperform even cheap paper cones and that the best solution would be a silk cone speaker. Crutchfield suggested a Blaupunkt self-powered sub, but I wasn't really planning to drop another $200 and, though that would be a nice addition, I don't think its necessary to get the performance I was looking for. They also never were able to address why the system sounded weaker at the low end than the two factory stereos. The system has plenty of power. Its only 18 watts RMS but its a VERY STRONG 18 watts.
So question 1 - Any idea why the system I described would be beat out by acceptable but average factory stereos?
Question 2 - Assuming I need to add to the system would a decent 6.5 in the rear do the job or should I start saving for that sub? Remember that its a small truck so not much power should be necessary.
Thanks for any advice or suggestions!

Answer 1

1. I think the term "beat out" isn't what happened. You have to look at what you replaced. Most factory systems are designed with the cheapest materials possible (i.e. paper cone, foam surround). Looking at those speakers, they are considered "one way" because just the cone is the speaker. So, why do factory speakers give you more bass. To be honest, I don't know the real reason but I can tell you in my experience it tends to be because of the design of the speaker. It is a full cone, with no cutout for a tweeter. Again, in my experience, that tends to give the speaker a better bass response.
Now, look at the speakers you replaced them with. You probably have 2 to 3-way speakers with a tweeter in the center. That alone accounts for you lack of bass. Most aftermarket manufacturers design their speakers to work with a full aftermarket system, amps and woofers included. So the speakers inside the car are designed for mids and highs, with little emphasis on the low end, the speakers just can't reproduce it as deisgned, or not very well at the least.Even if you amplify the speakers and push them to their max, I don't think you will get what you are looking for.
2. Going off answer 1, yes a sub will solve your problem, not a pair of 6.5"s. Being that it is a truck, there are plenty of applications out there to fit even something as small as an 8" (10" if you're shopping at BB). 
When I advise someone about speaker replacement, the first thing I tell them is they are going to lose some bass. It's just a matter of fact. What's nice about a car stereo system is you can piece it out, meaning you can do it a step at a time and get to a point where you are happy. That could be just a new head unit up to a full system and all the combinations inbetween. 
Hope that helps. Good luck with your sound.