Ok, so I recently fried my amp - exclusively my fault and I did know better. But I was never really happy with the configuration I had so I decided to upgrade. I had a Apline MRP-M500 with two Memphis 15-PR10S4 10 inch subs in a ported Memphis enclosure (15-PE2X10). Decent, but did not get loud or deep enough. I might ought to mention that I have a 2000 impala with a factory head unit and use a cassette adapter so I can use my Archos 605 as the signal source. (A cheap no name cassette adapter is what caused me to burn up my amp as the signal it delivered was a lot louder and distorted - like I said: my fault for cranking it when I knew better.) I also use a Soundstream BX-20Z bass processor because of the factory head unit. I'm pretty content with the factory head unit and I do not want to go through the effort involved in changing it out - in my case I would have to move it to the trunk because you can't completely dispense with it. Factory HU isn't likely to get stolen either. I have installed 20 sq ft of Second Skin's Damplifer Pro - did help with trunk rattle, but I need a lot more - the so called 'trunk pack' is only about 1/3 of the material necessary to do my trunk.
On to the new setup - I just ordered a Kicker 10ZX1000.1 amp and will order a pair of RE Audio SEx10d2's very soon - so the amp and speakers are not open to being changed. I considered stepping up to 12 inch subs this time, but decided against it as I've heard that 10's are 'punchier' and I'm not out to gain any tickets from the police, I just want it to sound good in the car. The original plan was to just stick the RE's in my existing Memphis box, but I'm not sure that would be the best thing to do. I do like like loud, but I also want it to sound good.
So - sealed or ported in my 2000 impala? Also I'm not much in to making my own boxes. I've done it before, but truthfully I think I'd rather pay to have one made if necessary.
For future reference, you heard wrong. Cone size does NOT change the sound of the sub.
I'd recommend ported, more output and it can sound almost as accurate as sealed when the box is built properly. On that note, get at bassman3 on here for a good quality box at a very competitive price.
And finally, 1995 called and wants it's tape adapter back. No seriously, stop using those right now. You're absolutely killing your sound quality with that. Put a real HU in there or at least get a hard-wire antenna adapter. It will sound a thousand percent better.
Having a custom enclosure is easily taken care of, just ask Dustin, he is the custom enclosure builder around here, atleast one of them. You can check out his site DLdesignz.com and can reach him through the site or at email@example.com. He makes top notch enclosures at awesome prices. Secondly if you don't want your subs to be too loud outside the car, you could sound deaden the trunk, etc to keep the sound waves in the car. This will also increase output for you inside the car and just help everything to sound a little nicer overall. The myth about 10s or 12s, or whatever sounding different is simply that, a myth. The more important factor to look at is a sub's specs. For example a ten inch sub that has nice excursion and has a good efficiency rating would get louder and lower than a 12 inch with less excursion and weaker efficiency. The enclosure a sub is in is also going to play a large part in how a sub will perform and sound for you. Sealed will give you a flat response while ported is going to give you a peak with roll off. Depending on your listening style will determine which is best for you. The main benefit of going with 10s is taking up less room for an enclosure. I'd recommend going the largest in sub you could, as long as you have room to house the proper enclosure. If you're only looking to take up a certain part of your trunk though, or overall weight is a concern, then of course go with whichever sub fits your needs. The amp and subs you got though are a good choice in brand and should do very well.
Larger subs will typically have a few differences (electrically speaking) due to more driver mass (lower Fs and F3) and more wire winding due to larger voice coils, but yeah as far as things like "a bigger sub will hit deeper but they aren't as tight" is a myth. What makes a sub tight or sloppy is the BL curve of the driver, which represents the strength of it's motor. A big sub can have excellent motor control and sound much tighter than a cheaply made and poorly designed smaller sub. As for hitting more deeply, a larger sub will tend to have more output so it will appear to hit more deeply, simply because the amount of air it can move in comparison to a smaller sub at low frequency is noticeably greater, but both subs are capable of moving air at frequencies at or below the range of human hearing.
Typically I prefer to stick with a single sub, as large as reasonably possible in the given application.