jFarley, I'm using the DX Tools Quantum phaser.
WBBM (along with the other Chicago blowtorches) are very difficult to null at my location, maybe because I'm battling groundwave+skywave, I don't really know. WBBM normally reads S9+35 on the Perseus but I'm able to get it down to S9 and that's good enough to hear some weak audio sneak through on 783, as I'm hearing again right now as I type this. These are really the only stations that I have trouble with phase-nulling. Other big guns like 1030 WBZ Boston, which throw a S9+30 signal my way, are no match for the phaser. I can usually phase them right down into the dust. Here's a nice example of this, when I caught Oregon on 1030 while phasing-nulling WBZ: http://www.youtube.com/watch?v=Rea6p96rVTI
One more example of phasing: I have a strong S9+45 local on 850 that I was able to phase down in order to catch Spain on 855. This clip shows my local before and after phasing to reveal the weak signal on 855:
http://www.youtube.com/watch?v=GtllxHT1v6EIt's a great tool for medium wave!