Yes, Richard, I was made aware of the two different types of holdback and takeup tensions by my brother, John Stephens, who developed the Stephens Multitrack (from 8 trks on 1" to up to 40 trks on 2") which is a capstanless deck (or decks) using a servo loop optical readout to drive the tape via the feed reels and takeups. The decks are still being used throughout the world for mastering due to their excellent signal to noise and other qualities (i.e., lack of tape warp, etc.). Also, analog still is preferred by many "golden ears".
Rod,
I'm sure you know, but I'd like to point out that in constant torque (as opposed to constant tension) machines, this speed variation was variable throughout the tape pack.
Classic Ampex machines were mostly constant torque. There was a constant tension option for certain versions of the AG-440.
Cheers,
Richard