I've always kind of wondered...if you were to digitize the D5 source and compare to your digital "back up", might it be possible to isolate the delta for an EQ profile of what the tape is doing to the sound? I always wished there was a Maxell XLII plugin I could throw on top of a source to sweeten it like the old days.
You could use a matching EQ - one that analyzes the frequency spectrum of one source and applies it to another, and get close frequency response-wise. But frequency response probably varies somewhat with level, and there are other non-linear things going on such as tape and head compression/saturation, and potentially unwanted stuff like wow, flutter and noise. Could emulate that with some tape emulation plugin. The question becomes how closely do you really want to emulate the behavior of the D5 - do you want a true warts and all emulation or just the aspects of it you find sonically attractive?
Somewhat OT- Similarly I've long thought someone should market a little device that vinyl audiophiles could use to play digital recordings through their turntable cartridges. A little output device that drives a phonograph pickup cartridge to emulate vinyl playback absent a physical record groove or disk. Essentially a small low-power cutting head that wiggles a stylus rather than cutting a groove. I suspect this would emulate much of what folks like about vinyl playback and the user can choose whichever cartridge they prefer. Perhaps offer a couple versions, one a fully contained unit with signal in/out, another with just an input and the driving head as output, something you set on or attach to an existing turntable base for use with the same cartridge and arm for playing records. Maybe such a thing exists already,.