The D1 have the old Scan Speak 8513 (with some modification) that I'm quite familiar with and like it very much! That's probably be the reason for the changes in transparency,as the mid-bass for both speakers is same series (Seas Excel).
Also good to know that you've found the bass of the D1 great for it's size!
Also good to know that you've found the bass of the D1 great for it's size!