Paul Vnuk Jr wrote a very comprehensive review of the DTP 640 REX for the Recording Magazine.
Summary: "First up in the versatility department, the DTP 640 REX houses both a standard dynamic capsule as well as a small-diaphragm condenser capsule on top of it, with the two capsules in perfect phase alignment. This mic offers a plethora of mixing choices [...]. Not only do you get the choice of the frequency settings, but also blending the capsules in a mix offers endless possibilities, because with any two mics a 2 dB change in relative level can yield big sonic changes.
Overall I was a bit surprised when listening to the dynamic mic by itself in its flat state, as it really did not sound like any of the other current mics on the market [...]. It's actually much fuller and, dare I say, more vintage sounding -- because unlike most of the above models it has no midrange scooping, nor is it as highly hyped in highs or lows as most kick-centric models.
However, blending the two mics together is where the magic starts. In simple terms, the dynamic is your boom and the condenser captures a clean kick hit in a way a dynamic won't.
Inside the kick, right up on the head by the beater, the flat response was interesting and sounded like a big, thumpy vintage '60s kick with no front head... which was interesting as I had the front head on and the drum pretty tightly muffled. It was a thick and rubbery, almost AKG-ish sound."
Conclusion: "The DTP 640 REX is a fabulous and full-sounding mic capable of many kick drum colors. While it is not as hyped and specific as many of the modern kick mics we are used to, it hints at flavors and combinations that are quiet expressive and unique. This is not a kick drum mic you would get bored with easily, and the best part is that all of this versatility can be had for a mere $299 street price, which is only slightly higher than some of the usual suspects."