It depends on which jklabs box you're talking about. Jon at jklabs built many "DVC" boxes that do have a preamp included as well. He also built the "ECMS" box -- only 2 ever built that I know of, one for AKGs, one for Gefells -- and the ECMS box did not provide gain. It was just ck6x cap >jklabs collette (presumably with FET, etc) > jklabs ECMS box, which acted as the mic bodies and provided the capsule bias voltage, but did not provide any gain.
I ran his AKG ECMS box for years -- very nice solution for AKG actives. But as Brad notes, it sounded a bit different to me than AKG 480 bodies. Not a huge difference, but still different. Reminds me more of the sound of using jwilliams modded 460's.