ICE upgrade help

hi all thanks for reading....

I have just started to get bits together to install new speakers in the front doors rear parcel self and amp them all up... i have tried halfords but they are useless for help and decent stuff and i just feel bass junkies try to sell u stuff u just dont want or need...

first problem i am having is with the door speakers i have been told that u can only fit 5.25" (13cm) in the fronts but have read u can fit 6.5" (16cm) in the front if you buy speaker adaptors... i have looked for these adaptors but i can only find adapotors that allow the fitting of 5.25" (13cm) speakers if anyone has a link for the 16cm adapters it would be greatly appreciated...

the next problem i have is that halfords guide and staff both said i could fit 6x9's in the rear on the parcel shelf... after ordering i checked to see how easy it was to access the rear speakers to find round speakers and not alot of space for adapters and 6x9's i then did a bit of reaserch and found that u could not fit 6x9's without major modding of the parcel shelf which i dont really want to have to do ( even auto leads website and sales staff say the adaptor for the e36 rear speaker would enable me to fit the 6x9's then i hear of other people that it would fit ..... any advice here would be appreciated here as i dunno what to get for the space as i dont know the size limits width and depth....

i have 2 12" vibe slicks 1200 watts each and want the bass from the subs to be felt better in the car i was advised by a guy from halfords (different branch) that i should mount them to the back of the back seats and cut holes in the back seats has anyone got any better ideas ? or is this even a good idea ?

after i have done these upgrades i am stumped for what else i can do with the stereo is there anything else i can do to upgrade the stereo ?

thanks for reading and any help will be greatly appreciated.....