Edit: Also, for those interested in the idea of diminishing returns on DA, it comes from the fact 1 double attack will have a smaller increase on your dps the higher your DA is before adding it. If you have no qa/ta, 1 da will always add on average 1 attack per 100 swings.
If you do 1 damage per swing, in 100 swings with no DA you will do 100 damage. If you add 1DA, that damage goes up to 101, a 1%(101/100) increase from 100.
If however you had 50 da your damage would be 150 in 100 swings. adding 1 da at that point increases your damage to 151, is only a .67%(151/150) damage increase.
In both of these situations, you have added 1 damage, but in the 2nd it is a lower % increase to your total damage. Because of this, other stats might pull ahead.