Personally, you should scrap the quad-pods for the bass idea. 8" subs in those cheap plastic enclosures won't sound nearly as good as they would in a sealed MDF (medium-density fiberboard) box. In my opinion, you'll be disappointed if you put 8" subs in those quad pods and expect good bass. Without getting too technical, there isn't enough airspace in them to produce proper frequency ranges for bass at any level, and unless the 6.5" and 8" holes are sealed from each other you're going to get alot of sound distortion, even at low volume. A good alternative would be to use 8" coaxial speakers (with bass blockers spliced into the wiring for them) to negate loud bass; they'd primarily be used for mid-range sound. Look into a 6.5" component set instead of a 6.5" coaxial set also; component sets produce much clearer high to mid-frequency sound.
While I'm thinking about it....you're planning to run an external amplifier for all this equipment, right? Most aftermarket CD/MP3 players can only support about 100W power; push it too hard and you'll certainly cook the deck and potentially have a fire. Trust me!
A decent alternative from a separate subwoofer box (i.e. a MDF enclosure) is a bass tube with Bazooka brand being the most effective. They've come a long way over the years - they're compact, have an on-board amplifier, and produce nice bass levels.
From experience, a stereo with great sound comes at a price. It boils down to how good you want it to sound v.s. $$$ you're willing to spend. A ballpark figure for a good-sounding stereo is $600ish...CD deck ($100-$200), amplifier ($100), wiring kit ($25-$30), 6.5" component set ($60-$100), subwoofer and box ($100-$200). That's if you're buying everything brand-new and from lower-cost places (Ebay!), and installing yourself. I'd also watch craigslist for subwoofer/box/amp, great place to score these items for cheap if you decide to go for used equipment. A professional stereo shop will empty your pockets if you buy from them and have them install; beware! Self installation is easy, just takes some elbow grease, determination, and a basic idea of how to read wiring diagrams/installation guides. I'd be happy to help where I can
My current setup was just under $600, and sounds terrific!
Alpine deck - $120 (Crutchfield.com)
Kicker 3.5" coaxial set - $50 (removed 4x6" dash speakers and installed there)
Kicker 6.5" component set - $75 (Ebay)
Kicker 10" sub and box - $50 (craigslist!)
Kicker 600W 2 channel amp - $150 (Ebay)
6.5" sound wedges - $100 (Ebay)