You mean large lightboxes is better than ulbrellas ?
The reflections you will see on the surface of the paint/varnish are reflections (mirror images) of the lightsource(s). When the lightsources are small, the reflections will be bright and small. When the lightsources are larger, the reflections will be larger as well, and less specular (locally clipped to white hotspots) but rather somewhat smoother (depending on the paint technique, although such large works are probably not painted with a 'painting knife').
The better method would be to use polarized light and a polarization filter (at crossed angles) on the lens, which will reduce the visibility of the lightsource reflections to virtually nill. However, due to the size of the artworks it may be difficult to fit such large lightmodifier areas with polarizing foil, especially if one doesn't do such hings more often. One may want to use smaller lightsource areas, which would also call for longer light to subject distance to reduce light fall-off over distance.