the purpose is to get some bass???? out of the front speakers????

dude!! that's what the rears are for! keep the bass on the rear and the highs/mids on the front for a balanced system. I'd say go with a nice set of 4" on the front and 5.25 or 6" speakers on the rear for mids/bass.
I run 5.25" Power Acoustiks on the back.. so far not too bad. I just couldn't get the 5.25 JL Audio's to fit the OEM speaker covers from the GTS, the Power Acoustiks fit the cover pretty much perfectly. Everyone with a GTS knows what I'm talking about.. damn rear speakers with the clip on covers built in.