If you're getting decent recording levels with this setup, then I'd agree a preamp may not matter with those mics/power supply, however, if your levels are consistently low, a preamp will help.
Here's why:
In digital recording, each bit holds 6db of dynamic range. A 16 bit recording device can therefore hold (theoretically) 96 db of information. If your levels are peaking at -40, then you're really only using 10 bits out of the 16 bits available to you (96 -40 = 56... 56/6 = 9.333). Adding gain in post will raise the overall levels, but you're not gaining any information. A preamp will allow you to push the levels higher (say, peaking at -4 instead) meaning you are using all 16 bits and effectively capturing more dynamic range.
Hope this helps