My eclipse 8443 has time alignment already. Alot of the high end units do. DIY comps are definitely a cheaper way to go, but since I can get DLS stuff for a very reasonable price, that is why I went this route. I currently have a set of 6.2 if anyone is interested (shameless plug).
The best way is with a multi meter and 60Hz test tone disc. Like Thomas said disconnect the other speaks, disconnect the sub, connect the multi meter to your speaker outputs, if running bridged, connect one channel only. Set your HU to at least 75% volume (my hu doesnt clip till max volume...
Arcs are great amps, I ran the 4150 and 2500, but wanted more power, now I am running Tru Technology T03 line with burr brown upgrade. Thinking of getting there tube amps now... oh yeah, yes they are good amps.
Just get a set of DLS Iridiums 6.3's and call it a day, use the passive crossovers until your ready for the next level, and go active. Mine sound great, midbass in the doors, mids and tweets in the kicks.
Take a look at the image dynamics IDQ 10" great sub, that will keep up with your type of music, also if you want to go to a 12", you can do an Adire Shiva.
First you will need to find out what subs you want to run first and build the enclosure to the manufacture's specs. For subs that require small enclosures and are good for rap,rock and country, I would look at Image Dynamics V3 series or Adire Audio Shiva. My friend builds custom enclosures for...
Check the gains on the amp, sometimes reversing polarity on the tweets softens the image. Install is very important. So dont assume your 600 dollar speakers are going to sound good if you dont do the install properly. Also the crossover setting should be at highest 80Hz, where are the speakers...
I agree with the Koda 8 or even Image Dynamics V3, but definitely go sealed. That will give you the best SQ possible. As for amps start off with a JBL, when your to step up to nice SQ amps, go for Tru Technology, Zapco or DLS. Tru makes class A amps as well.