42low, I am pretty sure English is not your first language and so your comments are not coming across very clearly.
I too think here's the problem of language barrier.
Now, what I have to say MIGHT be a challenge to your opinion but it is what I and many top technical people in the industry believe to be the case.
1) Any competently designed audio amplifier OF SUFFICIENT power will sound the same as any other good power amp and that has been accepted for decades. Many times the tweaky, beardy subjectivists have argued "The XYZ sounds thin and distorted. The YZX is smooth and faster" Mind you, they change their preferences every few years about the SAME designs!
2)The sound quality of a loudspeaker is not affected by the impedance chosen. Most "hi fi" speakers are nominally 8Ohms but there are/have been 6R units. One of THE most respected speakers ever made, the Rogers LS35/A was originally at least, 15 Ohms. Some early very HQ speakers were of very complex impedance and a sod to drive. The early Spendors blew one or two "lesser" power amps! Modern amplifier technology at monitor prices and quality levels are virtually bomb proof.
And lastly, the interior of a car is a bloody awful place to judge the finer points of sound! Even if it is not moving (when the noise floor will be about 80dBSPL!) the tiny tin box is not conducive!
Dave.
.... cause this is exactly as i (try too) say.
As long as the speakers ohms are higher than the amps ohms are as much as the watts are higher the sound get's better/deeper/more power. In reverse the amp needs to deliver more which it quickly can't, with lack power in the actual sound. The 6 ohm you've mentioned are mostly (if not always) designed next to 6 ohm amps.
I may not be a music business technical, but i've done my lessons on electronics in school with good results. And those same calculations made me blow away my friends. And i can tell you that the levels and sounds we reached, and the differences between in the cars were perfectly to be heard (and to be seen at the car's body and accessories )
Start calculate with the ohms, watts, voltages and amperes and you can no other than say i'm right. (or let a specialist calculate for you)
In this case, as i said no different, the 4ohm speakers on the 6 ohm amp can go perfectly right if the amp feeds enough watts (at least about 1,5 times the speakers need), which in many cases is not.