I’ll start my answer by saying that I personally do not like bent over barbell rows. I do NOT use them myself and when I have a client use them it is normally when they train in a home gym and have a limited access to back training equipment.
THAT HAVING BEEN SAID they can be a good assistance exercise for deadlifts. But not because they are great at stimulating the lats and mid back (they are inferior to many pulling/rowing variations in that regard). But rather because they are effective at strengthening the lower back by having you maintain that bent over position under load. In other words they can help the deadlifting by strengthening the lower back and specifically its capacity to maintain a solid neutral position under load.
With that in mind, which barbell row variation would be best to increase the deadlift? The one where you can hold the most weight without changing the torso angle and while still loading the lower back.
If you use a position where the torso is parallel to the floor (e.g Pendlay row) you likely cannot use as much weight and as such it is less effective for strengthening the lower back.
If you stand-up and have your torso somewhere between 90 (parallel to the floor) and 180 degrees (torso perpendicular to the floor) you will be able to use more weight thus put more tension on the postural functions of the lower back/core. BUT if you stand up too much you unload the lower back.
So the ideal torso angle when doing barbell rows to strengthen the deadlift would be the same angle you would find yourself into when the bar is just below the knees when deadlifting (since this is the weakest position and where most people lose their position).
If you look at these positions in a deadlift. Position 1 (similar to a Pendlay row) would be too low, you wouldn’t be able to use as much weight.
Position 3 (and 4 obviously) would be too high; the angle reduces the amount of stabilization / fixation work the lower back and core must do. Not to mention that it is an inferior position to work the lats an d mid-back (more of a trap pull).
So somewhere close to position 2 (maybe a TAD) higher would be the best position to row from.
Which is pretty much how Eddy Coan did barbell rows.
I believe that the biggest benefits to the deadlift will be from not changing the torso angle throughout the whole set.
NOW if you were using barbell rows to maximize upper back development (lats and mid back( I prefer a lower torso angle, close to being parallel to the floor. And if you want to focus more on the upper region (traps, rhomboids, rear delts) then you could use the same angle as Dorian Yates who used a more upright position. This allows the use of more weight but reduces lats involvement.
Note that Yates actually used different torso angles. With his lighter sets he used a lower angle and with his heavier ones a higher angle. Probably didn’t do I on purpose but it’s actually quite smart.