My recommendation would be to get some midrange drivers without tweeters. Woofersetc.com
has a pretty good selection for any price range - just be sure to order two if they don't specify that it's a pair. 2 ohm speakers will draw more power from your amp, so that's your best bet as long as you have a decent quality amp.
Stuff the soundbar with polyfill, and if you can afford it, layer it with sound deadening like dynamat or something similar. Set your amp's low pass filter as high as it can go, most amps will let you cut 200htz and above, but it may be higher; the higher you set it, the more midbass you will get out of the rear. Keep in mind that good low end response will be nearly impossible with 6.5" drivers... they are really designed for midbass and rear fill for a full system with a subwoofer.
Tweeters right behind your head are annoying and really aren't doing anything other than screwing up your soundstage. If you just want a bit more bass, than a midrange woofer is the way to go.