6x9's don't maintain the physical characteristics for reproducing sounds properly.
Mid speakers can be designed for mid-range or mid-bass. To accurately produce all the way down to 20hz physics says you need a 12" driver. A 6.5" will usually take you down to about 60hz w/a 24db slope but it is still driver dependent...meaning did they design the mid to be used as bass or mid-range? Bass frequencies actually extend all the way up and through 250hz.
To give you an idea of the frequency range needed for a tight kick drum you would add at 800hz, subtract at 200 and 60hz. Matching your drivers is important if you're worried about quality. If you just want bump in your ride I would suggest an Alpine Type-R(pretty accurate for a mass market speaker) run at 4ohms and 300 to 600watts total. Btw, running 600watts won't gain any more noticeable performance than 300.
If you want a suggestion for 6x9...I refuse
