speakers...6x9's or 6 1/2"?

I agree. I have allways done my own stero replacement/upgrades on my cars. First rule of thumb, when it comes to speakers, bigger is truly better. The speaker has an easier time reproducing the sound. A big bass speaker (6X9) will produce the lower notes not only easier, but with more clearity and range. The 6-1/2 round will do a good job, but will be a bit pale by compare to the 6X9. The 6-1/2 is actually an excellent choice for the front door.

The only way a 6-1/2 inch speaker could out perform a 6X9 in the rear deck is ethier and/or both of these 2 reasons;

The speaker is a better quality speaker
The speaker is part of a componet system (Base/mid seperate from the mid/trebbile speakers) Componets sytems of any decent quality will have a cross over box to direct the sounds to the proper speaker of the system.

Secondly, when you upgrade your stero, the stero part known as the head unit (The actual stero reciever/disc/mp3 etc... unit in the dash) will have only so much power. Even if it has 50 watts of power, I would seriously think of adding an amp to power the speakers.

While your saying, 50 watts is alot, trust me when I tell you, this is the other place where bigger, or more power, is truly better to have and not use rather than use all of the power available from a lesser powered unit. Which would be the head unit in this case.

When pushing the sound level up, the best sound is when you can turn it up to what ever level your ears are OK with and still have tons in reserve. Clearity remains great at low levels but can break up on high levels. If your system is good for you at 3/4 or less of the full volume potenial, then you have put togther a good system.

If you need to go to 3/4 full level or better to get the loudness you want, your under powered and your giving up sound quality.

It's also easiest to do a whole system by one manufacturer, like a total Sony system, or a total Pioneer system. More speakers the merry!