Check your Environment settings on the render tab. I believe by default the 'Environment Mode` maybe set to 'Dome and Scene' or 'Dome only' with a default low resolution HDRI on the dome. The 'Draw Dome' property is probably set to 'Off' so you don't see the dome in the render.

I suspect most of the light in your scene is coming from this default HDRI and your spot lights are probably not bright enough to add much light to the scene.

If you set the 'Environment Mode' to 'Scene Only' that will turn off the HDRI dome. Don't be surprised if your scene then renders very dark.

http://docs.daz3d.com/doku.php/public/software/dazstudio/4/referenceguide/interface/panes/render_settings/engine/nvidia_iray/environment/start

You can turn up the intensity of your lights, but make sure you do this in the photometric sections. A simpler way is to change the 'Tone Mapping' on the render tab. You can turn the ISO up until your get a render you like. You can use the "Nvida Iray" display style rather than the default 'Texture shaded' in the preview window to show you the actual lighting rather than doing a render. You can then change light intensities or Tone Mappings settings and see the results quickly, even it you do not have a Nvida graphics card. Just switch back to texture shaded once you get things setup.

http://docs.daz3d.com/doku.php/public/software/dazstudio/4/referenceguide/interface/panes/render_settings/engine/nvidia_iray/tone_mapping/start