Why are you so set on doing density work as your primary means of back training? Are you trying to get bigger, increase your max # of pullups, or simply trying to get stronger? It'd be easier to give advice if I knew your goals.
Most of the authors on T-Nation recomend training a given muscle group more than once a week. As you have it set up now, your only hitting your back only once, albeit very hard. Have you thought about using a method such as grease the groove, where you do a few sets of pull ups everyday? Also, in regards to training back, pullups are very important, but they don't serve to counterbalance all the pushing work you get in a westside routine. In order to do that you need to focus on doing more rows, supine chinups, and possibly seated sled drags. The guys at westside know what they are doing, and they set up the program the way they do for a reason.
I remember reading in a westside article, that they use timed sets with set weights as part of their accessory work, and as feeder workouts to help them recover from their primary upperbody workouts. You might consider using a density style workout where you do antagonistic supersets as your accessory work on your DE bench day