most of the good back beatings are not a result of the load you use but the way you handle that load. if you are contracting the back muscles you want to work effectively against that load, you will "feel" it where you want or get the desired level of satisfaction in your workout.
i had to humble myself due to my use of straps. yes i could use quite a bit of weight on pulldowns, rows etc, but when i started using my natural grip, (the weights went down) but the degree of control i had in contracting the target muscles went up. this is probably due to the lighter weight initially, but the degree of intensity went up as well. there are still times i have to stop because of failing grip. its individual for everyone. i choose to be more of an advocate of training everything so i dont have to isolate certain things. (forearms, etc) keep in mind, rows, lat pulldowns etc are movement specific exercises, and not muscle specific, so there is no way to truly isolate THE muscle you really want (lats) so best bet is to work the muscle complex as a whole and make the most of the exercise by bringing up any weak parts of the movement.
as a side note on the gravitron. give this a shot. at the top of the movement concentrate on squeezing your lats for a pause at the top. ex: you are at the top, pull your shoulder blades back and down and visualize your tricep area crushing an apple (or favorite object) against your lats. you may have to have more assistance at first due to grip but if you arent feeling something by staying real strict in your form....then...i dont know. hehe.
the assisted pullup is awesome for people who cant do full on pullups or want to do pullups real strict. keep in mind there are those doing regular pullups that shouldnt. thier form is heinous and they only see the need to go from point A(the bottom) to point B(chin above the bar) without regards to shoulder position, etc. learn the right way and you will leave the others behind once you gain the strength.