To clarify, there's an 'Ambient Strength' setting under Daz Default material, and in fact most common materials. Essentially this setting 'brightens' the object. Setting Ambient Strength to 100% will, in most cases, make it fully bright however the scene is lit. In some cases it's used to artificially light surfaces which aren't covered well by 'normal' lighting, in other cases it's used to make some surfaces stand out more. In the case of Lux, it's probably not there because the emphasis is on realistic lighting rather than 'faking it' with the use of ambient. Edit: Added an image to illustrate what I mean.

Most of my general stuff is done in 800x800 to start with. That goes for both 3Delight and Lux. Of course, the problem comes that if I want a transparent background it's more difficult to get light to bounce around the character. Putting in a plane works for getting light to bounce, but the problem is keeping it out of the frame and still getting the same effects. In the real world, people can't stand against a void, so there's no real-world equivalent situation that can help with that.

In those situations I tend to add more lights, which probably isn't helping my render times any. I'm still very much used to a 3-point lighting system which I tend to adopt for most of my work in Daz. Sometimes it's hard to break old habits.