It's not really the volume (# of reps) that is important if you want to build muscle.
Generally, it takes even only as little as 4 reps for a load to stimulate hypertrophy, as long as the load is significant.
Hypertrophy really isn't more about volume, but about load (that is, if the case were "high volume vs. high load", load would be the winner).
So, Ruhl, if you want to build muscle, it's not about how many reps you do for a weight. Chances are, if you do 15-20reps, that means the weight you are using is far below the weight you can actually use.
Remember, at 80-90% of your 1RM, all fibers are recruited and it takes just a few reps to do the job (i.e., to stimulate hypertrophy). By using a weight you can do 15-20 RMs on, you limit yourself to using a weight that is perhaps only 40% of your 1RM (just an approximate).
I hope this helps

-JV