No two manufacturers make sets that sound the same under any conditions. So, mixing EdSets and J&M, for example, your EdSets wil increase in volume more slowly than the J&M's but will continue gaining volume without distortion well into the highest volume settings. The imbalance is caused by one set being lower resistance than the other and drawing more of the amplifier's power, but one set may be more efficient or may 'max out' quicker and the volume imbalance will be very noticable.

It's best to match headsets, but if you don't you may find that a volume control on the 'loud' seat will help, or just moving the 'loud' speakers off center an inch or more will fix the imbalance for no cost at all.