6x9 all the way.
Speakers are not 100 % efficient. Each speaker will lose a certain amount of power just trying to move.
So you need to get as much power to the speaker as you can to get output.
Again- More speakers does not mean more decibels- unless you want tweeters screaming at you. Tweeters require little energy, mid range requires more and bass the most of all. Mids will be underpowered every time bass is called for- The bass will rob the mid range output straining to produce the bass.
MUST: get a head unit with the ability to cut the low end frequencies aka a built in "cross-over" (in this case you want to be able to cut 60hz and below). That size speaker in that style enclosure would be lucky to hit 70hz so this reduces power draw.
An alternative is a "bass blocker" rated for 60hz cut off (basically a capacitor you put on the speaker wire between the head unit and speaker), but they may draw just as much power as not having one - not sure.
Also I *think* the smoothness of "roll-off" will not be there when using a capacitor/bass blocker vs having it built into the head unit. Essentially the head unit cut at 60 would be gradual diminishing of volume from approx 80hz down to 60 whereas the capacitor would be sharper cut.
To drive the original point home:
If you have 20 watts would you rather give that to 20 speakers or 2? What would be louder then? Remember: speakers have a certain amount of loss or inefficiency. Its "sensitivity".
So get a speaker with the highest "sensitivity" rating you can find. This will give you more output/Db as it is a more efficient speaker. (The higher the sensitivity Db rating, the more efficient the speaker is).
There are a bunch of car audio no-nos here:
Lack of power, lack of separation of front and back waves (sealing front of speaker/ "air around you" from back/"cabinet". Also the L/R speakers share a common cabinet. Not the best.
If it were me:
2 6x9s or 2 6.5s
Small d class amp (optional depending on other powered farkles and what I had left in stator output)
Foam gasket to seal large gaps in fairing (if possible - never handled a fairing like this up close)
Sound Deadener (CLD type resonance Deadener) on insides of fairing to reduce vibration/resonance/Db loss. Just cover 20% of the surface area.
This is more important if the fairing /cabinet is pretty well sealed.
Poly fill (inside of a throw pillow) put inside fairing to make speakers "see" larger cabinet volume and this play lower frequencies. (With netting/ screen to keep it out of head unit aka stereo). Only if I can pretty much seal the fairing enclosure. Otherwise it's not going to do much.
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