Without more cone area and an enclosure you can get but so much. I have hertz HSKs, used to have Eclipse components. Something's gotta give, can't have your cake and eat it too. Dial back on the gain or volume dial and they will probably go a bit lower than what they are now without reaching their mechanical limits. Dial the gain in to give them full power and more or run it full blast, and MAYBE if you deaden the doors well 60-65 HZ and clean is doable. Without good door prep 70-80HZ may be your lower limits. I am not quite sure how you tolerate them full range, must be some incredible mids. Every set of normal 6.5s that Ive ever tried to run full range sounded like crap at anything above moderate volume on bass-heavy music but not horrible on instrumental stuff.
Alot of people mix up the meaning of bass and midbass when they're even trying to explain what they want. They 'say' they are lacking midbass when what they really want is for their front stage to thump like it has a sub, and I bet if you listened to most peoples' setups who pose this kind of question they're probably getting about as much as they can reasonably expect out of their front stage as it's installed. Unless you invest in a REALLY good install and tuning most 6.5 mid range/midbass drivers can't be expected to go too much lower than the 'attack' portion of the bass beat which fools our ears into thinking that the whole beat is coming from up front. The 'reverbration' portion of the bass beat is where they will start to struggle if not designed, tuned, and installed to go really low. If you're not getting both of those aspects of the bass beat all you're getting is a sharp thud without any life or depth to it. Less noticable in instrumentals that don't really go that low but more apparent in rap where the bass beats tend to reach much lower frequency ranges.
Had a friend with some 6.5 inch subs. Still nothing like a big sub but it surprised me how low they could go in a proper enclosure. But these were specifically designed drivers made for a certain range of frequencies put in an enclosure so they did a better job of it than a 6.5 midbass/mid range in an otherwise unmodified door. Why do you not want a sub. And who told you your lack of deadening is not the problem. Add mass to them panels so your midbass frequencies are not turned into wasteful sheetmetal vibrations.