[Accord Man]

To have pounding bass, yes you will need sub(s)..

A crossover is meant to block certain frequencies, example: you dont want woo much low bass going to your 6.5s, so you'ld use a highpass crossover to let only the frequencies above a certain range to be played byt those 6.5s. The opposite is also true (lowpass). You dont want high frequencies going to your subs, cause that'll cause unwanted distorsion, thus decreasing the maximum potential output of your subs. There's also such a thing as a passthru, which lets the frequency between an A freq and B freq to be played.

A 4ch amp is commonly used to power 4 cabin speakers (front L R and back L R). A 2ch amp is commonly used to power 2 cabin speakers (priority for the fronts) OR to power 2 subwoofers (less common) OR both channels are bridged (if possible) and it powers the subs(s).

Are you satisfied with your cabin speakers? I dont know what you have in the front, but the Legacy's that you have in the rear are sorta crappy.. But again, if your happy with the sound, then great..

If your only worry is bass, then you dont need to worry about the cabin speakers.. You just need to figure out what type of subwoofer setup you want (you'll obviously need a suitable amp as well), and you'll be pounding.