I'm an owner of the Audison Voce 3 way set. As far as sound goes and what you could expect from a nice set of components, these are up there in quality with just about any other nice 3-ways...
The limited output on the low end is likely attributed to tuning, power, install, or expectations.
I was running a Audison Due (260 x2) to my 6's and they were solidly installed and tuned with my BitOne... what was off in the equation was my expectation... I was coming from having dedicated 8" mid bass woofers and expecting my 6.5's to do the same for some odd reason...
You will likely not gain anything from switching to another 6.5" mid bass, because the displacement will remain the same and the power handling/power delivery will remain virtually unchanged, as will the install more then likely.
If you are stuck to using a 6.5, beef up the install, increase the power you are giving them (within reason), and dedicate a ton of time to tuning... I was playing my voce 6.5's at 60 hz with a 24db slope and they did pretty good... but I still ended up swapping them for Morel Elate 9's because I couldn't adjust my expectations and love for insane midbass.