Using a slow shutter speed to control portrait light quality may not be such a great idea. It will not work with strobe as the "shutter speed" with strobe is the flash duration. Flash duration (a few thousandths of a second perhaps) is very short, so the subject isn't likely to move much at all while the flash is on.
It usually cannot be counted on for the subject to move about the "zero" point. The light will cast a longer, deeper shadow as the light moves to a shallower angle. and vice versa.
It is much more reliable and repeatable to move the light or make it relatively larger.
Light quantity (exposure) is controlled by shutter speed, f/stop and ISO. These are all set in the camera and have no effect on what hits the lens.
Light quality ranges from diffused to specular, largely controlled by the size of the light related to the subject. It is also affected by the subject - diffused or specular. This can be changed by powder on the subject (more diffused) or oil (more specular).
A simple exercise you can do yourself is to photograph a diffuse and specular subject with the same light setup. I used a racket ball for diffused and a glass Christmas tree orniment for specular. Both blue. Use a bare bulb on each and see what happens as you change the light to subject distance. Use a soft box and change the light to subject distance.
You may see that a bare light very close to a small subject will look diffused while a small soft box far away will look specular. The Sun on Mercury is much more diffused than it is on Pluto. An oily model may look specular while the same model with a dab of powder on those shiney high lights will look more difused.
I don't think the answer is just one thing. Like many other things, lighting can be complicated.