Perhaps people slags 6x9 cause it's very hard to mount them in the dash.
First of all, backspeakers are crap!
Do you sit on a concert back towards the band? No, I don't think you do
Backspeakers just confuses soundimage.
And what comes in the 6x9. I've never heard a pair of 6x9 wich sounds good. Those "3-way" or even "5-way" speakers just don't work how they supposed to work. Lot's of shitty sounding bass, tweeter shouting on your ears and midrange sounds like shit.
Second, If I had 6x9 speakers, I would remove the tweeter and midrange from them and use them as freeair subs IF they could repeat low frequencys. I've never seen pair wich can so.. And the bass which comes out from the 6x9 sounds almost always like crap with the other sounds it produces.
If someone makes GOOD pair of 6x9 midbass or sub, I think I would test them in my car. 6x9 is quite big speaker so it's quite hard to mount in saxos door but what the hell, I've mounted 6,5". Why wouldn't I trie 6"x9"
And what comes to the 6x9 in sub use, 6x9s conediameter (or what's the word. Surface area or something) along with the xmax won't be big enough to produce low frequencys with big enough soundpressure.
This is just my opinion.
Sorry for the bad english..