I posted this on Ethan Winers forum as well. His response: "Yes, it is probably one or more deep nulls". And he also says to measure the room.
A good track to test in room bass response is Rebecca Pidgeon Spanish Harlem. The bass line should stay nice and even. If some notes jump out and some disappear then you have a problem. (I found this trick from Bob Katz either from his website or his book on Mastering)