The example you posted is certainly made with diffuse sidelight, very likely a window, since it is just possible, I think, to see the shadow of a window bar across the easel - that's the trouble with pesky daylight, unless a window faces due north there's always the chance of the sun coming out or going in when not wanted. In this picture, Newman seems to have relied on the light walls to fill in the shadows. "Rembrandt" lighting is of course nothing more than diffuse sidelight but often with a dark background so that the overall effect is a lot moodier. As Gordon Moat has observed, a reflector is a very handy thing - there are high-tech reflectors (Lastolite, etc.) which fold up into a 15" bag or so but open out to a good size. The 1000 W lamps you have bought sound pretty useful, as long as they have sufficient cooling to ensure that the softboxes don't catch fire and that the lamp bodies do not get too hot to handle! As you probably know, if you place identical lights so that one is 1.4x the distance from the subject that the other one is, you will get a 1-stop difference - twice the distance means 2 stops. You can also vary the lighting ratio even with one light and a reflector - the closer the lamp (in a key light position) and the further away the reflector, the higher the lighting ratio, and vice-versa.
Regards,
David