I'll buy anybody a pint who can find me either a SQ or SPL competition car - at any level - that has achieved success with 6x9s.
I could give you endless information on why 6x9's on the rear shelf - especially when a sub is below - is a bad idea. I can give you exact theorys on why this doesn't work well, I can give you opinions of some of the highest regarded ICE installers and competitors around the country on why it's a bad idea. But unfortunately I fear it will fall on deaf ears.
Just remember - just because something sounds louder, or there is a slight improvement in any area (regardless of the negative affects the change has) it doesn't mean you have spent your money well, have the best setup possible for your money or even have a good, balance system.
Rear speakers in general aren't needed, but oval speakers are just laughable when trying to convince somebody that they will improve sound quality. And as for putting them on the parcel shelf, that's just a joke for a number of reasons. Firstly, it shouts 'LOOK AT ME!! I'VE GOT ICE IN MY CAR!!! STEAL ME!!!' to anybody who happens to glance in your direction. Secondly, you're mounting them onto glorified cardboard. Yup. Cardboard.
When was the last time you saw a home audio speaker mounted in cardboard? Any speaker for that matter, home, PA, anything. Who in their right mind would think flexible, bendy cardboard is a good surface to mount speakers on?
Also answer me this... to those who are convinced that slapping a set of rear speakers on your parcel shelf will make your car sound better... why did you not upgrade the front speakers first? You sit in the front, right? The vast majority of the sound you hear will come from those, right? So why add speakers at the opposite end of the car?
Number69 - You use the arguement of money as to why 6x9's are such a great idea. Let's say you have £100 to spend. That £100 would buy a basic but nice set of front components and a Dynamat door kit. I catagorically, 100% promise that this upgrade will improve the sound more than 6x9.s
And if you want to take it one step further and you're looking at buying a stealth shelf as well as your 6x9's for £100. That's around £200 spent. For that money you could buy a small amp to run your front speakers, as well as the door kit, giving improved bass, improved midbass, improved sound dynamics and more of a powerful feel to the car. Clarity, quality and definition are all improved.
Ok, so you don't want an amp for the front speakers? Fine, take that £100 and buy yourself a sub, box and amp combo. Many ICE retailers off 'bundle' packages including everything you need for that sort of money. Then you have given yourself the added bass you desire, for the same budget.
So what other reasons do we have for 6x9's? Sound quality? Nope... oval speakers are sonically inferior to round speakers. So, bass? Nope, a well sound deadened set of front speakers will achieve better results for the same money. Ok, rear fill? You already have rear speakers in very good locations that are of idea quality for rear fill and rear passengers. Running out of reasons now...
I am not debating that you have experienced a change in sound in your car, of course you have. But simply chucking in a set of rear speakers onto a sheet of cardboard at the back of your car, so the sound is aimed at your rear windscreen and then bounced off onto the back of your head, is never, ever going to give as much of an improvement in sound as upgrading the front speakers. Ever. Even a cheap set of front speakers will outperform a set of 6x9's at double their price in this situation.
I know you lot already have your rear speakers, but the point in this thread - as with every other thread on this forum - is to share advice. If I were to make a post about the wonders of using an eBay 99p resistor in my LCR people would be quick to provide 101 reasons why I shouldn't and why I was wasting my money. This is the same. I am simply telling you that you have misspent your money and you could have achieved far greater results for the same, if not less money.