Help w/ questions on my proposed setup - db651 vs. mm651 - and 8" sub

The DB drivers are ok but are not as transparent and detailed as the MM. There is more midbass/bass from the MM series as well. Go to onlinecarstereo.com, they have the MM for cheap. I would highly recommend the components over coax. Night and day difference.

MM are $209, DB are $149. Its only 60 bucks and the MM's are well worth that difference. They will reach down lower and blend better with the sub. I have another idea for your amp solution.

Honestly, from a SQ standpoint I have to recommend that you try to spend some extra money for an amp that lets you go active. That is so you can get rid of the passive xovers and use the filters on the amp. By doing this you get up to 50% more power to your speakers. This is because passive xovers absorb tons of energy from the amp and turn it to heat. Also passive xovers are not as clean.

The polk audio PA D5000.5 is only $350 and and would be an all in one solution.

In this setup
Channels 1 and 2 would drive your woofers from the 6501 (100W because they are 2 ohm)
Channels 3 and 4 would drive your tweeters from the 6501 (70W because they are 4 ohm)
Channel 5 would drive the sub (400w RMS at 2 ohm)

I know that you may think that 100W going to the woofers alone may be overkill. It is definitely not, it gives you tons of headroom so that you never get close to distortion or clipping.

I'm suggesting that you not power your rear speakers at all or at most power them a tad with deck power. Rear speakers are bad in car audio. Think about it. If you listen to music at home, even on a 5.1 system. Where do the vocals come from? They come directly in front of you and the music spreads from the left to right. You never hear vocals behind you in this setup and never hear vocals from behind you in a movie theater either. Competition vehicles rarely use rear speakers because they just mess things up. You'll get much better performance and imaging by just using fronts.

Its your choice whether you put the tweeters down by the woofers or up in the A pillar. In general A pillar tweeters will sound better if they are angled opposite headrest. If you do put them up high you will likely just turn them down a bit by adjusting the gain on the amp, this can help with harshness. Tweeters down low do have their advantages however by blending better with the mids but the stage will be a little lower without EQ to help out. There are trade offs either way. Going active how i suggested lets you fully customize your tweeter level compared to the woofers. When you use the passives, you only have a choice of 0db, -3db, or +3 db, which is limited. Plus with the active option you can play with the xover frequency to see where it sounds best.