braveheart, as Keith has pointed out, gain-matching is only useful when the subs are identical. If the subs are dissimilar, gain matching doesn't work. In the case of dissimilar subs, some other method of setting the gains must be used. Level-matching at the LP is one way, and may or may not be the best way. Another technique for setting the levels is the Geddes technique as described here: http://mehlau.net/audio/multisub_geddes/ Both methods assume that there is only one *important* position, the one you measure the levels at. Two subs placed in different positions in the room will certainly measure different SPL's at different positions in the room. Therefore, if you set the levels based on one position, they will be wrong at every other position. What if you have 2 rows of seats? If you set the levels for the middle seat in the front row, they will be wrong for all the other front row seats and all the back row seats. If I had dissimilar subs, I would use the Geddes technique and measure at every important listening position, and then average them, (i.e, use the "spatial average" for setting the levels.) Having said that, I would avoid using dissimilar subs if at all possible.
Nonetheless, IF the subs are identical, there are some distinct advantages of gain-matching. The point of gain-matching the gain structure of the entire system, (receiver trims AND the gain pots on the subs), is to ensure both subs are working equally hard, and driving the exact same energy into the room from different locations. A 6 dB offset of one sub over the other means that the higher set sub is using 4X the excursion ...at all frequencies... of the lower set sub. It's also using 4X the amplifier power. If you run your subs anywhere close to their limits, the higher set sub will run out of headroom 6 dB earlier than the lower set sub. It will limit the overall system and you won't take advantage of the full capability of the lower set sub.
If you don't care that one sub is working 4X as hard as the other, and you never listen to your system anywhere close to it's limits, then level-matching or the the Geddes technique can work fine. But, if you want to ensure you can use the full capability of the subs you paid good money for, gain-matching is the only way to ensure that.