Mono sub channel summation

When we're talking about active crossovers, setting the gain is pretty straightforward when we're talking mid-to-high frequencies; just give the stereo channels a flat response. But I have a question about what happens when you have a mono sub. Do you make it so that the sensitivity of the mono channel is (up to) 6 dB higher because there is only a single channel, or match the sensitivity with the stereo channels because the summed signal will automatically be at a higher voltage and result in the appropriate higher SPL? I'm leaning toward the latter but would like to hear others' opinions/facts.
 
As with a lot in home stereo, there may be a theoretical answer but just a starting point in fine-tuning. For example, results may depend on the degree of correlation of the stereo sub signals which the engineer chose.

With two subs, I find about the same loudness for stereo bass as for mixed bass. And have never yet decided which makes for better bass.