There are two "full range" drivers in each side of the storage compartment once you complete the work via the kit's instructions:
Being "full range", they specialize in nothing and therefore compromise lows, mids, and highs. Lows are relatively non-directional so the door speakers do an OK job of filling them in. Mids are OK with just the kit's drivers. I personally found the highs to be lacking, so that is where the tweeter idea comes in (highs being very directional). Not being an audio engineer, all this is just IMHO.
Looking at the picture, you can see that there is a nice flat space on the outboard side of the divider panel to attach a surface mount tweeter on.
A good audiophile or electrical guru's opinion on how best to select the tweeters type (and how to connect them) is most desired - I've yet to purchase them! (FYI, the existing "full range" drivers are 4-ohm units {by Becker} and as you might see in the picture they are wired in parallel).
As to how the kit impacts the sound, I found it to be
much improved by addition of the basic kit, especially with the top down. However, if you're really discerning about sound, you may not be satisfied unless you go the separate drivers (tweeter, mid, woofer, bass tube?) and aux amp route. After all, this kit is really just adding a few speakers to your existing factory radio...
