I was suggesting you need both a one channel (mono/sub) amp for the subs and a smaller 2 channel amp to power your door speakers. Even on a budget system I can't recommend anyone run speakers off the HU amplifier. I suggest an external amp to power them. They will sound much better and get much louder. The Hifoncis you listed would work great for your subs, but you will still need a 2 channel amp for your door speakers.Larger 6 1/2 speakers will sound better and give more bass than a comparable 5 or 4" speaker. IMO, 6 1/2" speakers will give you the best "bang" for your buck. IMO, the only reason to go with smaller speakers is if you can't make them fit, and usually with a little bit of modification most people can make 6 1/2's fit. It's worth the effort. Depth is usually more of a problem than width, and a small wooden ring "riser" will usually take care of both problems.
Your front door speakers provide most of the sound/music you hear. They do ~98% of the work, so it's important to get good speakers, install them well, seal & deaden your doors and provide them with plenty of amplifier power. Most of the bass you "hear" comes from the door speakers (some people call this mid-bass), and smaller speakers, like 4"-5" speakers just can't produce any usefull bass.
Your subs only work on the lowest 2 % of the sound/music. But it takes a ton of power and speaker cone size to do it. This is the bass you "feel".
Read the stickied thread at the top of the "Speaker" sub forum on how to seal and deaden your doors.
