The D20's were a late addition to the original Decade series I believe. I purchased, and still own, a pair of D10's. However, I seriously considered the D20s as well at the time. My understanding is that the drivers for the original Decades were custom built to Ty's specifications. I also understand that they are no longer available.
If you haven't already reached out to Ty, I would suggest doing so as he will likely be able to suggest alternatives to the original drivers.
If you haven't already reached out to Ty, I would suggest doing so as he will likely be able to suggest alternatives to the original drivers.