The SPL meter will tell you much more than your ears can in terms of level setting. Yes, use the receiver's tones and write down the measurements from the various locations you are considering for the sub. You are looking for the best SPL readings, though you might also consider doing it with a range of tones rather than just a test tone - different locations will give you different readings at different frequencies. It isn't impossible to do by ear, but your ear can be fooled much more easily than the meter.