6000K is only the "heat color" of the light, not brightness - the higher this number, the bluer the light.
The brightness in the Lumens or LM. If the bulbs were sold as a pair at 12000LM, it really means they are 6000LM each.
I have used a single CREE with 12K LM and I found it very bright. Is it maybe the reflector? Adjusted correctly? Is there a shield over the LED element? Maybe try the non-LED Sylvania (incandescent) and see if that makes a difference. If not bright, the issue must be related to the aftermarket bucket reflector or directional adjustment.