Careful analysis of all of my basses shows that most strings are balanced for output but the D string is marginally quieter than the rest. This seems to transcend brand or string type. It's not like the string is dead or sounds bad, it's just got a smidge less output than the others (also accounting for more dB to make the bigger strings equally audible as the smaller ones). I guess this is why a lot of string sets make the D string way more taut than the others (to get the bigger mass that causes that tension to go up) but it doesn't seem to fix the problem either. The origin of the problem? In my mind it's got to be mostly the fault of the radiused fingerboards. The D string has the least mass of all the strings raised further from the pickup because of radiusing so suffers the most (I guess). It's a niggling issue for sure but I wonder if anything could be done about it?