I've came across a number of so called midbass drivers like CDT EF-6
and HD-6. Where do they fit on the spectrum? How does one use it -
IB or some kind of small enclosure (6L). Where should they be placed?
Rear-deck? Trunk? Kick-panel? So how do people use them usually?
A pair of full range in front, then a pair of mid-bass at the
rear-deck and a sub in the trunk?

One of the thing I don't understand is how to utilize the rear deck.
Currently I'm running a pair of Infiniti kappa 625 but because it's
facing upward, they never gives good high to the rear passengers
anyway. So I'm thinking what if I change them to mid-bass driver, at
least that will enhance the listening for everyone in the car, even
though the rear passengers will be missing highs.

Ernest

I've came across a number of so called midbass drivers like CDT EF-6
and HD-6. Where do they fit on the spectrum?

The midbass region.

How does one use it -
IB or some kind of small enclosure (6L).

Usually IB is sufficient, assuming the baffle is adequately sealed. But I
have done small boxes before with these speakers, and perhaps surprisingly,
have enhanced the response.

Where should they be placed?
Rear-deck? Trunk? Kick-panel?

Ultimately, wherever you've got the space for it. Oftentimes it's best to
put it in the front kick panels or doors, but some people have good results
in the rear doors or rear deck. I've got a pair in the front doors and in
the rear doors, complementing the 4" and tweeter in the front and dome mid
in the rear.

So how do people use them usually?
A pair of full range in front, then a pair of mid-bass at the
rear-deck and a sub in the trunk?

Typically, yeah. If your midbass will be handling up to, say, 500-700 Hz,
then you can obviously get away with a smaller midrange driver. I've done
installations where I've used just a small coaxial for the mid/highs.


One of the thing I don't understand is how to utilize the rear deck.
Currently I'm running a pair of Infiniti kappa 625 but because it's
facing upward, they never gives good high to the rear passengers
anyway. So I'm thinking what if I change them to mid-bass driver, at
least that will enhance the listening for everyone in the car, even
though the rear passengers will be missing highs.

That's typical, actually. Many folks will install just a midbass, just a
midrange, or both in the rear deck and leave out the tweeter.