Let me offer my point of view.
I don't agree that setting the levels with just a meter is always a better choice. First, I don't listen to the music with a meter, I normally use my ears
. Second, the whole thing of meters being properly calibrated, what weighting curve to use - A or C can make it even more difficult.
One the other hand setting the level by the ear by comparing the loudness of 40Hz-80Hz pink noise on the sub with 500Hz-2000Hz on the mains can be misleading as well because our ears have very different sensitivities at these frequencies.
So I use a combined approach. First, I use the meter to set the levels and I usually set the sub 3-4 dB hot.
Second, I set the crossovers on the mains and the sub at 80Hz and use a sweep frequency tone from 15Hz to 150Hz . I use the DVE DVD but I'm sure there are plenty of other sources.
The thing I'm looking for is a smooth transition at around or 80Hz and here is when I use the ears. Since I'm comparing the sub and the mains at the same freq and at the same time the ear's sensitivity should play a lesser role. If the transition is not smooth with the loudness deepening or increasing I change the sub volume control accordingly.
Hope it helps