I guess opinions are like @ssholes. We all have them and they are all hairy... or something like that.
Your system is actually one of the best designs. If I understand your description correctly you have 2 pairs of components (front and back) and a single 10" sub. No deck has enough true power to adequately run components (or at least good ones). Don't bother with trying to block out bass. The separation is the whole point of components and most reproduce bass very accurately. Let the crossovers do their job and send the right frequencies to each speaker in the component sets.
Several companies are making 5 channel amps now where the 5th channel has a sub crossover built in. You can probably pick up a decent one in the $300-500 range. Your deck would need 2 pairs of preamp outputs to allow you front to rear fader control. The sub input can come from any single preamp output and most amps will probably do this internally. Viperoni is right, the staging is the most important part of the install.
Low bass is all mono and omnidirectional so the sub can be placed pretty much anywhere with the same effect. For low end, one 10 should be plenty boomy and still tight unless you want that low rumble that rattles everything loose on those dumb@ss lowriders you see scraping around while the idiot driver literally can't even hear half of the music he's listening to. -gratuitous rant
Mid-bass is the punchy part you feel and you probably want that coming from your front component sets (like Viperoni said) although I wouldn't block it from your rears either. This is what you "feel" when listening to fast jazz or metal and some choice funk.
The tweeters require the most thought in placement as they are totally directional. I like putting them near the mids, maybe angled in door panels. If they are up high you just get an earful of tweet. You want to hear the highs, but wherever they are, your attention will be drawn toward them (like a small insect buzzing near your head).
Not sure, but I'd guess that sub control on your deck is made to work with a feature on xplod model amps. Go to a good quality car audio shop to research what you want and then buy it elsewhere. Soundwerks is about the best if you have them there. Try to talk to an installer and not a salesperson if possible.
If you can't find a good 5 channel amp you could go with a 4 channel for the components and a mono sub amp just for the 10.
When buying an amp look for a high amperage fuse, heavy duty terminals, big cooling area, and a quality build. Don't just go by the numbers on the paper. Manufacturers can make the numbers do whatever they want. I get a kick out of seeing Jensen amps rated at 400W with a 12A fuse. Lets see, 12 Amps X 12 Volts = 144 Max watts that the amp is capable of pulling (although it would probably fry if it did).
