What 6.5 mid is best for low end bass?

[PaulD 10+ year member]

you don't need a ton of airspace for a 6.5"

 

[95BlackGA 5,000+ posts]

Foam speaker baffles make for a little better response.

http://www.crutchfield.com/g_762/Foam-Speaker-Baffles.html?tp=660

Best way would be to get/make kick panels.