I've used the 507c with the ACSS Vulcan reticle. I'm not sure how comparable it is to the 507k, but whatever glass is used in the 507c doesn't have the greatest low light transmission. To see the target clearly through one tube, I need to hit it with an IR illuminator (not super intense, just a Surefire Vampire light).
However, I have no trouble seeing the 507c reticle. With dual tubes it's not too bad even without the illuminator, since one tube picks up the reticle, and the other picks up the target. I have to adjust the focus on the tubes a bit from my usual setting to get the clearest picture of the reticle, since the reticle image is meant to appear to be something like 25 meters away, and I usually set my focus a little bit farther out than that. It's not a huge difference.