I doubt that Sargent could have drawn like Prud'hon, since even Bouguereau, Gérôme or Lord Leighton didn't draw that well ... perhaps not caring to as much since their drawings were more functional as building blocks in the process of painting. I've had the good luck to have been permitted to examine one of Prud'hon's greatest drawings that is in storage at the Clark Museum, and it frankly was beyond incredible. The supposed tightness that you think you are seeing from a photo is really delicately crafted layers of inky black shadows that carefully build his forms to a breathtakingly beautiful emotive three dimensionality reminiscent of Rembrandt's greatest etchings.
I would rather own a great Prud'hon drawing to a painting my most artists (not Bouguereau or Lord Leighton of course).
Sargent drew great emotive drawings using a "looser" technique, but it was no greater. I also don't agree that it's harder to draw great loose than tight. Both methods are equally valid in the right hands, and both are incredibly difficult to achieve where comparisons of superiority of technique is really not as helpful as comparisons of finished products by different artists.
I havent' been able to follow all the posts, so excuse me if I seem to miss responding at appropriate times.