Metal surfaces are one of the most difficult things to photograph. You're on the right track with using diffused lighting but if, as you say, the diffusers you have are "taking away too much light" then you have to compensate for that by using a tripod and longer shutter speeds.
I've used white umbrellas with the light shining through them to light metal surfaces to good effect, but another method- that of bouncing the light off big sheets of white foam board worked even better. Angling the boards lets you taper the lighting so that you get better definition of the metal surfaces (particularly if there's any engraving on the surface) and strategic placement of a black tape or paper strip or two across the surface of the white board can break up the monotony of the smooth or brushed metal surface.
Another possible option is creating a light tent with white fabric draped around the subject matter but far enough away that it doesn't intrude into the frame and shine your lights through that. There are also ready made light tents or "sheds" made just for this type of "table-top" setup.