You are awesome, thank you!1.) the order is good, you may not even need a 4ch amp if all your going to get is the basslink. so i'd get the speakers and just see how you like that setup the balance should be decent volume wise.
2.) usually rattling, vibrations, etc contribute to the ability to perceive the origin of the bass (without these its hard but not impossible to use sound differences between the ears to get an idea where the bass is coming from) below 80Hz its hard or almost impossible to tell where its coming from, unless you have the rattling etc. this is a main reason why the 80Hz is a general starting crossover point between subs/component speakers.
3.) i just ordered from ae64 today. sent him my payment, i got an email saying it was shipped today as well. The cruise control is handled by the ECU (despite what uninformed people at the dealership will say: i had a guy try to tell me that i couldn't change the headunit cause everything was run through it.)
His site can be kind of confusing, so you'll need:
20 pin forward harness with 3 SWC cables (blue tooth phone interface i think)
axxess aswc-1 module (if you want the volume, mode, etc buttons to work)
metra ax-subusb (to retain stock usb location, his site also gives other options)
depending on head unit you can get a parking brake bypass from him (to allow eq settings and DVD while moving)
AUX Jack (if you want to retain the stock AUX location)
^^^but if you buy the ipod wire with your headunit they usually take up one of these, but if you email him he'll tell you what will/won't work for you.
4.) another option is you could get a decent 8" sub and just throw it in a small sealed enclosure. it will take up room (usually 0.25 ft^3 or less though).
if you have any questions about setting gains (which is NOT a volume knob, it's more like a voltage from the head unit (through the RCA's) sensitivity knob), or crossovers send me a PM