WARNING: This post contains speaker p*rn . . .
I agree that 4x10s are not great sound producers. I swapped the factory 4x10s for a set of Pioneer 4x10s 30 years ago and could not even tell the difference. A 40W per channel amp just to the rears helped but not much.
And, again true, 6x9s won't perform as well as a round speaker but you can still get a ton of sound out of them. Though I am now switching my Sony 1 DIN head out for a RetroSound unit, this sound system below sounds pretty amazing to my ears. My only disclaimer is I am comparing the sound now to how it used to sound . . . not to the most amazing sound system I've ever heard and, if you are a bumper buzzer, this won't be your cup of tea either but I don't get the whole bumper buzzing thing . . . I just love pristinely clear LOUD music.
I have these JBL GX302s up front in the dash mounted [under] the stock mount points since the coaxial centers are too tall for the covers to mount them on top. They just run off the 25W head power with 150Hz bass blockers so these just add fill sound but they sound SO much better and produce SO much more sound than the factory set.
And these JBL GX962s are mounted on the rear deck running off a Sound Ordnance 110W per channel amp. I dial down the bass output of this amp by 6dB which helps the 6x9s perform even better because . . .
I put in a single 10" Rockford Fosgate powered sub in the middle right behind the rear seat. Just for those that actually don't know, you mount it facing rear [which seems a little counter-intuitive but that's how they are supposed to mount] and it sits perfectly against the angle of the rear seat. I think the sub no doubt makes the most difference but I am most surprised by how much sound will come out of those dinky front speakers.