Hello!???
Whatever happened to the excellent method of doing DB-one-arm-bentover-rows that former U.S. Superhvywgt. National Powerlifting Champ Terry Todd regularly used (and, performed by legions of old-time BBers for decades)!?? Terry did STRICT BB bent-over rows with 405 lbs. and DB bent-over rows with 200+ lb for reps....AND he had a HUGE lat spread in his prime!
In this time-tried-and-proven style, BOTH FEET are planted firmly on the floor, the free-arm braced on a sturdy bench, upper back positioned approx. parallel to the floor, and the DB rowed strictly to touching the DB plates to torso. The key term here is rowed-STRICTLY with no added heaving/help with the low back and hips/legs! Isolate the action to the lats and upper back!
With the current popular style of resting one knee on a bench in doing this exercise, you are putting a lot of questionable/harmful pressure on the knee joint in its contracted position, particularly with the heavier DB's like 120 lbs. plus. Why, in the gym, I've even seen a few guys whose knees have slipped off of the bench, and they then bounced their knee off the floor before they could regain their balance...OUCH!??
What's wrong with planting both feet firmly on the floor, supporting the torso in the parallel-to-floor-position with your free arm on a sturdy bench, and STRICTLY rowing the DB to the torso!??? Sounds like a lot simpler/safer/healthier method of performing this excellent lat exercise to me!