If 6.5's are what fit, and you don't want to modify the deck, That's what you'll want to go with.
My research indicates that 6.5's fit the rear deck and the front doors, and 4" fit the rear doors.
"All Treble and no Bass" will be an equalization problem with most any speaker in a sound system. A speaker that doesn't push much bass needs to have the treble turned down to balance the sound.
If you're running the stock head unit, that could be part of the problem, as it probably pushes 6 watts RMS, or something small like that. My "50x4" Pioneer is something like 22 Watts RMS, which is more capable of pushing aftermarket speakers with poly cones and they will sound better.
After I wrecked my car, I put the stereo in my Wife's Honda- 6.5's in the rear deck, 5.25 in the doors, and a Pioneer DEH-6500 (about 6 years old). No amp (yet), No Subwoofer (yet) but it sounds great once tuned.
So my advice is to get what fits. 6.5's can be great speakers. If I were going to buy speakers, and didn't want an Amp, I'd likely be buying the 50 dollar Pioneer 2 way speakers. They will probably be more "boomy" than component speakers designed to be used in conjunction with subwoofers and amplifiers.
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Andrew
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A man's character may be learned from the adjectives which he habitually uses in conversation.