There are far better forums to ask that question, many specifically on speaker design for DIYers, however, the most obvious answer would come down to phase angle introduced by your cross-over and the driver itself, such that the 6.5" wired in phase, is actually out of phase during the critical cross-over transition regions and is causing suck-out in the frequency response.
What Vienna Acoustics wrote is no more than marketing. I cone moves as a complete signal unit every bit as much as a flat surface. It is meant to impress people who don't know what they are writing is just market speak. Sort of like the Phase Technology guy claiming that his drivers are acoustically opaque to the back wave (and others are not), and yet his cone is only 3.1 grams (ignores the weight of the flat piece). If you believe that, I have some swamp land for you in Florida. Don't tell me in marketing speak, show me actual measurements that prove what you claim. If you can't show your driver has lower distortion, more even dispersion, lacks resonances, and can truly block the back wave, then it is just market speak.
Don't get me wrong, designing a good speaker takes work, but as you will see in DIY sites, people with limited physics knowledge, by building on the work of others, can achieve very significant results.
(Side note, there is technical merit in Dunlavy's time and phase aligned design, however, that does not necessarily translate into higher levels of listener satisfaction. It has technical merit, but it is not proven out to be psychoacoustically critical. <-- Not even 1 area of physics needed. Dunlavy didn't push the boundaries of any physics in his implementation and most of what was accomplished was "math", i.e. the mathematically modelling to achieve time alignment, but I digress).
What Vienna Acoustics wrote is no more than marketing. I cone moves as a complete signal unit every bit as much as a flat surface. It is meant to impress people who don't know what they are writing is just market speak. Sort of like the Phase Technology guy claiming that his drivers are acoustically opaque to the back wave (and others are not), and yet his cone is only 3.1 grams (ignores the weight of the flat piece). If you believe that, I have some swamp land for you in Florida. Don't tell me in marketing speak, show me actual measurements that prove what you claim. If you can't show your driver has lower distortion, more even dispersion, lacks resonances, and can truly block the back wave, then it is just market speak.
Don't get me wrong, designing a good speaker takes work, but as you will see in DIY sites, people with limited physics knowledge, by building on the work of others, can achieve very significant results.
(Side note, there is technical merit in Dunlavy's time and phase aligned design, however, that does not necessarily translate into higher levels of listener satisfaction. It has technical merit, but it is not proven out to be psychoacoustically critical. <-- Not even 1 area of physics needed. Dunlavy didn't push the boundaries of any physics in his implementation and most of what was accomplished was "math", i.e. the mathematically modelling to achieve time alignment, but I digress).